Pixel sensor having multiple photodiodes

ABSTRACT

In one example, an apparatus comprises: multiple distinct sets of photodiodes, wherein each set of photodiodes includes one or more photodiodes, one or more charge sensing units, and a controller. The controller is configured to: transfer charge generated by the one or more photodiodes in response to a different component of incident light to the one or more charge sensing units in order to convert the charge to voltages; perform one or more quantization processes of a plurality of quantization processes corresponding to a plurality of intensity ranges, wherein the one or more quantization processes quantizes the voltages from the one or more charge sensing units to digital values representing components of a pixel of different wavelength ranges; and generate a pixel value based on the at least some of the digital values.

RELATED APPLICATION

This patent application is a continuation of U.S. Non-Provisionalapplication Ser. No. 16/453,538, filed Jun. 26, 2019, entitled “PIXELSENSOR HAVING MULTIPLE PHOTODIODES,” which claims priority to U.S.Provisional Patent Application No. 62/690,571, filed Jun. 27, 2018,entitled “Digital Pixel Sensor with Multiple Photodiodes,” and which areassigned to the assignee hereof and are incorporated herein by referencein their entirety for all purposes.

BACKGROUND

The disclosure relates generally to image sensors, and more specificallyto pixel cell structure including interfacing circuitries fordetermining light intensity for image generation.

A typical pixel in an image sensor includes a photodiode to senseincident light by converting photons into charge (e.g., electrons orholes). The incident light can include components of differentwavelength ranges for different applications, such as 2D and 3D sensing.Moreover, to reduce image distortion, a global shutter operation can beperformed in which each photodiode of the array of photodiodes sensesthe incident light simultaneously in a global exposure period togenerate the charge. The charge can be converted by a charge sensingunit (e.g., a floating diffusion) to convert to a voltage. The array ofpixel cells can measure different components of the incident light basedon the voltages converted by the charge sensing unit and provide themeasurement results for generation of 2D and 3D images of a scene.

SUMMARY

The present disclosure relates to image sensors. More specifically, andwithout limitation, this disclosure relates to a pixel cell. Thisdisclosure also relates to operating the circuitries of pixel cells togenerate a digital representation of the intensity of incident light.

In one example, an apparatus comprises: a plurality of photodiodes, eachphotodiode being configured to convert a component of incident light ofa wavelength range to charge; one or more charge sensing units; one ormore analog-to-digital converters (ADCs); a memory; and a controllerconfigured to: enable the each photodiode to generate charge in responseto a different component of the incident light; transfer the charge fromthe plurality of photodiodes to the one or more charge sensing units toconvert to voltages; receive a selection of one or more quantizationprocesses of a plurality of quantization processes corresponding to aplurality of intensity ranges; based on the selection, control the oneor more ADCs to perform the selected one or more quantization processesto quantize the voltages from the one or more charge sensing units todigital values representing components of a pixel of differentwavelength ranges; store at least some of the digital values in thememory; and generate a pixel value based on the at least some of thedigital values stored in the memory.

In some aspects, the each photodiode is configured to, within anintegration period, accumulate at least a part of the charge as residualcharge until the each photodiode saturates, and to transfer theremaining charge as overflow charge to the one or more charge sensingunit after the photodiode saturates. The one or more charge sensing unitcomprises a charge storage device having a configurable capacitance. Theplurality of quantization operations comprise: a first quantizationoperation to generate a first digital value representing a quantity ofthe overflow charge received by the charge storage device configured ata maximum capacitance, the first quantization operating being associatedwith a first intensity range; and a second quantization operation to,after the residual charge is transferred to the charge storage deviceconfigured at a minimum capacitance, generate a second digital valuerepresenting a quantity of the residual charge stored at the chargestorage device, the second quantization operation being associated witha second intensity range lower than the first intensity range.

In some aspects, the plurality of quantization operations comprises athird quantization operation to generate a third digital valuerepresenting a time-of-saturation of the charge storage device caused bythe overflow charge.

In some aspects, the apparatus further comprises a light receivingsurface through which the plurality of the photodiodes receives theincident light. The plurality of photodiodes forms a stack structurewith respect to the light receiving surface such that the eachphotodiode is separated from the light receiving surface by a differentdistance. The component converted by the each photodiode is based on therespective distance between the each photodiode and the light receivingsurface.

In some aspects, the apparatus further comprises a filter array on afirst side of the light receiving surface, the filter array havingfilter elements positioned at a plurality of locations on the first sideof the light receiving surface to set a component of the incident lightthat enters the light receiving surface at the respective location. Theplurality of photodiodes correspond to a plurality of sub-pixels and arepositioned at the plurality of locations on a second side of the lightreceiving surface to receive the respective components of the incidentlight.

In some aspects, the apparatus further comprises a single microlens overa plurality of filter arrays including the filter array and configuredto project the incident light received from one spot of a scene towardsthe plurality of locations on the first side of the light receivingsurface.

In some aspects, the apparatus further comprises a plurality ofmicrolenses including a first microlens, the first microlens coveringthe filter array and configured to project the incident light receivedfrom one spot of a scene towards the plurality of locations on the firstside of the light receiving surface.

In some aspects, the one or more sensing unit comprises a single chargesensing unit. The one or more ADCs comprises a single ADC coupled withan output of the single charge sensing unit. The apparatus furthercomprises a plurality of switches, each switch coupled between aphotodiode of the plurality of photodiodes and an input of the singlecharge sensing unit. The controller is configured to: control theplurality of switches to transfer the charge generated by the eachphotodiode to the single charge sensing unit to convert to voltages; andcontrol the single ADC to quantize the voltages generated by the singlecharge sensing unit.

In some aspects, the controller is configured to: control a first switchof the plurality of switches to transfer a first overflow charge from afirst photodiode of the plurality of photodiodes to the single chargesensing unit to convert to a first voltage; based on the selection,control the single ADC to perform at least one of the first or thirdquantization operations of the first voltage to generate a first digitalvalue; control the first switch to transfer a first residual charge fromthe first photodiode to the single charge sensing unit to convert to asecond voltage; based on the selection, control the single ADC toperform the second quantization operation of the second voltage togenerate a second digital value; control a second switch of theplurality of switches to transfer a second residual charge from a secondphotodiode of the plurality of photodiodes to the single charge sensingunit to convert to a third voltage; based on the selection, control thesingle ADC to perform the second quantization operation of the thirdvoltage to generate a third digital value; and output, from the memory,generate the pixel value based on one of first digital value and thesecond digital value.

In some aspects, each of the photodiodes has a different full wellcapacity for storing the residual charge. The controller is configuredto: control the plurality of switches to transfer overflow charge fromeach photodiode of the plurality of photodiodes to the single chargestorage unit simultaneously to generate a first voltage; and based onthe selection, control the single ADC to quantize the first voltageusing at least one of the first or third quantization operations togenerate a first digital value; control the plurality of switches totransfer residual charge from the each photodiode to the single chargestorage unit at different times to generate second voltages eachcorresponding the respective residual charge from the each photodiode;based on the selection, control the single ADC to quantize the secondvoltages using the second quantization operation to generate seconddigital values; and generate the pixel value based on the first digitalvalue and the second digital values.

In some aspects, the controller is configured to, within a first timeperiod: control the plurality of switches to transfer overflow chargefrom each photodiode of the plurality of photodiodes to the singlecharge sensing unit at different times to generate first voltages eachcorresponding the respective overflow charge from the each photodiode;and based on the selection, control the single ADC to quantize the firstvoltages using at least one of the first or third quantizationoperations to generate first digital values. The controller is furtherconfigured to, within a second time period: control the plurality ofswitches to transfer residual charge from the each photodiode to thesingle charge storage unit at different times to generate secondvoltages each corresponding to the respective residual charge from theeach photodiode; and based on the selection, control the single ADC toquantize the second voltages using the second quantization operation togenerate second digital values; and generate the pixel value based on atleast some of the first digital values and the second digital values.

In some aspects, the one or more sensing unit comprises a plurality ofcharge sensing units corresponding to the plurality of photodiodes. Theapparatus further comprises a plurality of switches each coupled betweeneach charge sensing unit of the plurality of charge sensing units and acorresponding photodiode of the plurality of photodiodes.

In some aspects, the controller is configured to: enable a firstphotodiode of the plurality of the photodiodes to transfer a firstcharge to a first charge sensing unit of the plurality of charge sensingunits to generate a first voltage; enable a second photodiode of theplurality of the photodiodes to transfer a second charge to a secondcharge sensing unit of the plurality of charge sensing units to generatea second voltage; based on the selection, control the one or more ADCsto perform the first quantization operation of the first voltage togenerate a first digital value, followed by the second or thirdquantization operations of the second voltage to generate a seconddigital value; and generate the pixel value based on the first digitalvalue and the second digital value.

In some aspects, the controller is configured to, within a first timeperiod: enable a first photodiode of the plurality of the photodiodes togenerate a first charge in response to the incident light; enable thefirst photodiode to transfer a first overflow charge of the first chargeto a first charge sensing unit of the plurality of charge sensing unitsto generate a first voltage; and based on the selection, control the oneor more ADCs to perform the third quantization operation of the firstvoltage to generate a first digital value representing a firsttime-to-saturation. The controller is further configured to, within asecond time period: enable a second photodiode of the plurality of thephotodiodes to generate a second charge in response to the incidentlight; enable the second photodiode to transfer a second overflow chargeof the second charge to a second charge sensing unit of the plurality ofcharge sensing units to generate a second voltage; and based on theselection, control the one or more ADCs to perform the thirdquantization operation of the second voltage to generate a seconddigital value representing a second time-to-saturation. The controlleris further configured to generate the pixel value based on the firstdigital value and the second digital value.

In some aspects, the plurality of charge sensing units comprises a firstcharge sensing unit, a second charge sensing unit, a third chargesensing unit, and a fourth charge sensing unit. The one or more ADCscomprise a first ADC and a second ADC. The controller is configured to:control the first ADC to quantize a first voltage from the first chargesensing unit and a second voltage from the second charge sensing unit;and control the second ADC to quantize a third voltage from the thirdcharge sensing unit and a third voltage from the second charge sensingunit.

In some aspects, the controller is configured to store each of thedigital values in the memory.

In some aspects, the controller is configured to: control the one ormore ADC to generate a first digital value based on quantizing a firstvoltage corresponding to charge generated by a first photodiode of theplurality of photodiodes; store the first digital value in the memory;read the first digital value to compute the pixel value; control the oneor more ADC to generate a second digital value based on quantizing asecond voltage corresponding to charge generated by a second photodiodeof the plurality of photodiodes; overwrite the first digital value witha second digital value in the memory; and read the second digital valueto compute the pixel value.

In one example, a method is provided. The method comprises: enablingeach photodiode of a plurality of photodiodes of a pixel cell togenerate charge in response to a different component of incident lightreceived by the pixel cell; transferring the charge from the pluralityof photodiodes to the one or more charge sensing units to convert tovoltages; receiving, for each photodiode of the plurality ofphotodiodes, a selection of one or more quantization processes of aplurality of quantization processes corresponding to a plurality ofintensity ranges; based on the selection, controlling the one or moreADCs to perform the selected one or more quantization processes toquantize the voltages from the one or more charge sensing units todigital values representing components of a pixel of differentwavelength ranges; storing at least some of the digital values in amemory; and generating a pixel value based on the at least some of thedigital values stored in the memory.

In some aspects, the plurality of quantization processes comprises afirst quantization process to measure a quantity of residual chargeaccumulated at a first photodiode of the plurality of photodiodes beforethe first photodiode saturates, a second quantization process to measurea quantity of overflow charge transferred by the first photodiode afterthe first photodiode saturates, and a third quantization process tomeasure a time-to-saturation of the one or more charge sensing unitscaused by the overflow charge from the first photodiode.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative examples are described with reference to the followingfigures.

FIGS. 1A and 1B are diagrams of an example of a near-eye display.

FIG. 2 is an example of a cross section of the near-eye display.

FIG. 3 illustrates an isometric view of an example of a waveguidedisplay with a single source assembly.

FIG. 4 illustrates a cross section of an example of the waveguidedisplay.

FIG. 5 is a block diagram of an example of a system including thenear-eye display.

FIG. 6 illustrates block diagrams of examples of an image sensor.

FIGS. 7A, 7B, and 7C illustrates example operations by the image sensorof FIG. 6 .

FIGS. 8A, 8B, 8C, and 8D illustrates examples of arrangements ofphotodiodes of the image sensor of FIG. 6 .

FIG. 9 illustrates block diagrams of an example of a pixel cell.

FIG. 10 illustrates examples of charge accumulated with respect to timefor different light intensity ranges by the pixel cell of FIG. 10 .

FIGS. 11A, 11B, 11C, and 11D illustrate techniques for performingquantization.

FIG. 12 illustrates an example sequence of control signals to performlight intensity measurement.

FIGS. 13A and 13B illustrate block diagrams of an example of a pixelcell having multiple photodiodes and its operations.

FIGS. 14A and 14B illustrate block diagrams of an example of a pixelcell having multiple photodiodes and its operations.

FIG. 15 illustrates a flowchart of an example process for measuringlight intensity.

The figures depict examples of the present disclosure for purposes ofillustration only. One skilled in the art will readily recognize fromthe following description that alternative examples of the structuresand methods illustrated may be employed without departing from theprinciples, or benefits touted, of this disclosure.

In the appended figures, similar components and/or features may have thesame reference label. Further, various components of the same type maybe distinguished by following the reference label by a dash and a secondlabel that distinguishes among the similar components. If only the firstreference label is used in the specification, the description isapplicable to any one of the similar components having the same firstreference label irrespective of the second reference label.

DETAILED DESCRIPTION

In the following description, for the purposes of explanation, specificdetails are set forth in order to provide a thorough understanding ofcertain inventive examples. However, it will be apparent that variousexamples may be practiced without these specific details. The figuresand description are not intended to be restrictive.

A typical image sensor includes an array of pixel cells. Each pixel cellincludes a photodiode to measure the intensity incident light byconverting photons into charge (e.g., electrons or holes). The chargegenerated by the photodiode can be converted to a voltage by a chargesensing unit, which can include a floating drain node. The voltage canbe quantized by an analog-to-digital converter (ADC) into a digitalvalue. The digital value can represent an intensity of light received bythe pixel cell and can form a pixel, which can correspond to lightreceived from a spot of a scene. An image comprising an array of pixelscan be derived from the digital outputs of the array of pixel cells.

An image sensor can be used to perform different modes of imaging, suchas 2D and 3D sensing. The 2D and 3D sensing can be performed based onlight of different wavelength ranges. For example, visible light can beused for 2D sensing, whereas invisible light (e.g., infra-red light) canbe used for 3D sensing. An image sensor may include an optical filterarray to allow visible light of different optical wavelength ranges andcolors (e.g., red, green, blue, monochrome, etc.) to a first set ofpixel cells assigned for 2D sensing, and invisible light to a second setof pixel cells assigned for 3D sensing.

To perform 2D sensing, a photodiode at a pixel cell can generate chargeat a rate that is proportional to an intensity of visible lightcomponent (e.g., red, green, blue, monochrome, etc.) incident upon thepixel cell, and the quantity of charge accumulated in an exposure periodcan be used to represent the intensity of visible light (or a certaincolor component of the visible light). The charge can be storedtemporarily at the photodiode and then transferred to a capacitor (e.g.,a floating diffusion) to develop a voltage. The voltage can be sampledand quantized by an analog-to-digital converter (ADC) to generate anoutput corresponding to the intensity of visible light. An image pixelvalue can be generated based on the outputs from multiple pixel cellsconfigured to sense different color components of the visible light(e.g., red, green, and blue colors).

Moreover, to perform 3D sensing, light of a different wavelength range(e.g., infra-red light) can be projected onto an object, and thereflected light can be detected by the pixel cells. The light caninclude structured light, light pulses, etc. The pixel cells outputs canbe used to perform depth sensing operations based on, for example,detecting patterns of the reflected structured light, measuring atime-of-flight of the light pulse, etc. To detect patterns of thereflected structured light, a distribution of quantities of chargegenerated by the pixel cells during the exposure time can be determined,and pixel values can be generated based on the voltages corresponding tothe quantities of charge. For time-of-flight measurement, the timing ofgeneration of the charge at the photodiodes of the pixel cells can bedetermined to represent the times when the reflected light pulses arereceived at the pixel cells. Time differences between when the lightpulses are projected to the object and when the reflected light pulsesare received at the pixel cells can be used to provide thetime-of-flight measurement.

A pixel cell array can be used to generate information of a scene. Insome examples, a subset (e.g., a first set) of the pixel cells withinthe array can detect visible components of light to perform 2D sensingof the scene, and another subset (e.g., a second set) of the pixel cellswithin the array can detect an infra-red component of the light toperform 3D sensing of the scene. The fusion of 2D and 3D imaging dataare useful for many applications that provide virtual-reality (VR),augmented-reality (AR) and/or mixed reality (MR) experiences. Forexample, a wearable VR/AR/MR system may perform a scene reconstructionof an environment in which the user of the system is located. Based onthe reconstructed scene, the VR/AR/MR can generate display effects toprovide an interactive experience. To reconstruct a scene, a subset ofpixel cells within a pixel cell array can perform 3D sensing to, forexample, identify a set of physical objects in the environment anddetermine the distances between the physical objects and the user.Another subset of pixel cells within the pixel cell array can perform 2Dsensing to, for example, capture visual attributes including textures,colors, and reflectivity of these physical objects. The 2D and 3D imagedata of the scene can then be merged to create, for example, a 3D modelof the scene including the visual attributes of the objects. As anotherexample, a wearable VR/AR/MR system can also perform a head trackingoperation based on a fusion of 2D and 3D image data. For example, basedon the 2D image data, the VR/AR/AR system can extract certain imagefeatures to identify an object. Based on the 3D image data, the VR/AR/ARsystem can track a location of the identified object relative to thewearable device worn by the user. The VR/AR/AR system can track the headmovement based on, for example, tracking the change in the location ofthe identified object relative to the wearable device as the user's headmoves.

Using different sets of pixel cells for sensing different incident lightcomponents (e.g., different visible light components, visible lightversus infra-red light, etc.) can pose a number of challenges. First,because only a subset of the pixel cells of the array is used to measurea particular incident light component, the spatial resolution of theimaging can be reduced. For example, in a case where different subsetsof the pixel cells are used for 2D imaging or 3D imaging, the spatialresolutions of both of the 2D image and 3D image are lower than themaximum spatial resolution available at the pixel cell array. Likewise,the spatial resolution for imaging of a particular visible lightcomponent is also lower than the maximum spatial resolution available atthe pixel cell array. Although the resolutions can be improved byincluding more pixel cells, such an approach can lead to increases inthe form-factor of the image sensor as well as power consumption, bothof which are undesirable especially for a wearable device.

Moreover, since pixel cells assigned to measure light of differentwavelength ranges are not collocated, different pixel cells may captureinformation of different spots of a scene, which can complicate themapping between images of different incident light components. Forexample, a pixel cell that receives a certain color component of visiblelight (for 2D imaging) and a pixel cell that receives invisible light(for 3D imaging) may also capture information of different spots of thescene. The output of these pixel cells cannot be simply merged togenerate the 2D and 3D images. The lack of correspondence between theoutput of the pixel cells due to their different locations can beworsened when the pixel cell array is capturing 2D and 3D images of amoving object. While there are processing techniques available tocorrelate different pixel cell outputs to generate pixels for a 2Dimage, and to correlate between 2D and 3D images (e.g., interpolation),these techniques are typically computation-intensive and can alsoincrease power consumption. Similar problems can occur in the mapping ofpixel cells associated with different visible light components forreconstruction of a 2D image.

The present disclosure relates to an image sensor having an array ofpixel cells and can provide collocated imaging of different componentsof incident light from a spot of a scene, and to provide a globalshutter operation. Specifically, each pixel cell can include a pluralityof photodiodes, one or more charge sensing units, one or moreanalog-to-digital converters (ADCs), a memory, and a controller. Eachphotodiode of the plurality of photodiodes is configured to convert acomponent of incident light to charge. The controller can transfer thecharge from the plurality of photodiodes to the one or more chargesensing units to convert to voltages. The controller can also receive aselection of one or more quantization processes of a plurality ofquantization processes corresponding to a plurality of intensity rangesand, based on the selection, control the ADCs to perform the selectedone or more quantization processes to quantize the voltages from the oneor more charge sensing units to digital values representing componentsof a pixel, which can correspond to the spot of the scene. Thecontroller can also store at least some of the digital values in thememory, and generate a pixel value based on the at least some of thedigital values stored in the memory. The pixel values from the array ofthe pixel cells can represent the incident light received by each pixelcell within a global exposure period to support the global shutteroperation.

Various techniques are proposed to enable the plurality of photodiodesto convert different components of incident light from a spot of ascene. the pixel cell includes a light receiving surface through whichthe plurality of the photodiodes receives the incident light. In someexamples, the plurality of photodiodes forms a stack structure withrespect to the light receiving surface such that the each photodiode isseparated from the light receiving surface by a different distance. Asthe incident light propagates through the stack structure, differentcomponents can be absorbed and converted by the each photodiode based onthe respective distance between the each photodiode and the lightreceiving surface. In some examples, each photodiode can be formed in asemiconductor substrate. The pixel cell can be formed by stacking aplurality of semiconductor substrates comprising the plurality ofphotodiodes with the light receiving surface being formed on the top orthe bottom semiconductor substrate in the stack.

In some examples, the plurality of photodiodes can be arranged laterallyand having the same distance from the receiving surface, with eachphotodiode corresponding to a sub-pixel and configured to receiveincident light from the same spot of the scene. A filter array can bepositioned on a first side of the light receiving surface of the pixelcell. The filter array may include filter elements positioned at aplurality of locations on the first side of the light receiving surfaceto set a component of the incident light that enters the light receivingsurface at the respective location. The plurality of photodiodes can bein a single semiconductor substrate and positioned at the plurality oflocations on a second side of the light receiving surface to receive therespective components of the incident light. In some examples, one ormore microlens can be positioned over the filter array to project theincident light received from that same spot of the scene towards theplurality of locations on the first side of the light receiving surface,such that each photodiode, as a sub-pixel, can receive incident lightfrom that same spot.

The one or more charge sensing units may include a configurable chargestorage device and a buffer to convert the charge generated by theplurality of photodiodes to voltages. The configurable charge storagedevice may include a floating drain to accumulate the charge from theplurality of photodiodes to generate the voltages. An auxiliarycapacitor (e.g., a metal capacitor, a metal-oxide-semiconductor (MOS)capacitor, etc.) can be connected or disconnected from the floatingdrain to expand or reduce the capacitance of the charge storage device.A buffer can buffer the voltage at the charge storage device to increaseits driving strength. In one example, the one or more charge sensingunits may include a single charge sensing unit which can be shared amongthe plurality of photodiodes to perform charge-to-voltage conversion. Inone example, the one or more charge sensing units may include aplurality of charge sensing units each corresponding to a photodiode ofthe plurality of photodiodes and configured to convert the chargegenerated by the respective photodiode to a voltage. In both examples,the pixel cell further includes a plurality of switches, with eachswitch coupled between a photodiode and the shared/corresponding chargesensing unit, which can be controlled by the controller to control thetransfer of the charge from the photodiode to the charge sensing unit.In a case where the one or more ADCs are shared among the plurality ofcharge sensing units, each charge sensing unit can also include a switchcoupled between the buffer and the one or more ADCs which can becontrolled by the controller to select which of the charge sensing unitto provide an output voltage to the one or more ADCs.

The controller can control the one or more ADCs to quantize the outputvoltages output by the one or more charge sensing units. The one or moreADCs can be shared between the charge sensing units (in a case where thepixel cell includes multiple charge sensing units), or may include asingle ADC coupled with a single charge sensing unit of the pixel cell.The one or more ADCs can quantize the voltages based on differentquantization operations associated with different intensity ranges,which can increase the dynamic range of the light intensity measurementoperation. Specifically, each photodiode can generate a quantity ofcharge within an exposure period, with the quantity of chargerepresenting the incident light intensity. Each photodiode also has aquantum well to store at least some of the charge as residual charge.For a low light intensity range, the photodiode can store the entiretyof the charge as residual charge in the quantum well. For a medium lightintensity range, the quantum well can be saturated by the residualcharge, and the photodiode can transfer the remaining charge as overflowcharge to the one or more charge sensing units. The quantum wellcapacity can be set based on a bias voltage on the switch between thephotodiode and the charge sensing unit. For a high light intensityrange, the charge storage device in the one or more charge sensing unitcan be saturated by the overflow charge. The controller can control theone or more ADCs to perform different quantization operations for thehigh intensity range, the medium intensity range, and for the lowintensity range based on measuring the overflow charge and residualcharge.

Specifically, for measurement of the high light intensity range, thecontroller can control the one or more ADCs to perform atime-to-saturation (TTS) measurement operation by quantizing the time ittakes for the charge storage device to become saturated by the overflowcharge. The TTS measurement can be performed within the exposure time,and when the photodiode is coupled with the charge sensing unit and theoutput of the charge sensing unit is coupled with the one or more ADCs.The capacity of the charge storage device can be maximized by connectingthe floating drain with the auxiliary capacitor for the TTS operation.In the TTS operation, a counter can start at the beginning of theexposure time. The one or more ADCs can compare the output voltage ofthe charge sensing unit with a static threshold representing thesaturation limit of the charge storage device to generate a decision.When the decision indicates that the output of the charge sensing unitreaches the threshold, a count value from the counter can be stored inthe memory to represent a time-to-saturation. The TTS operation can beperformed when the light intensity is so high that the charge storagedevice becomes saturated and the output of the charge sensing unit nolonger correlates with the quantity of charge generated (and the lightintensity), which can extend the upper limit of the dynamic range.

For measurement of the medium intensity range in which the chargestorage device is not saturated by the overflow charge, the controllercan control the one or more ADCs to perform a FD ADC measurementoperation to measure a quantity of the overflow charge stored in thecharge storage device. The FD ADC measurement can be performed withinthe exposure period, and when the photodiode is coupled with the chargesensing unit which can temporarily store the overflow charge and convertthe stored charge to a first voltage. The one or more ADCs can comparethe first voltage with a first ramping voltage to generate a decision.The counter can start counting at the starting point of the rampingvoltage, and the memory can store the count value when the first rampingvoltage reaches the first voltage. The stored count value can representthe quantity of the overflow charge. In a case where multiple chargesensing units share an ADC, the ADC can compare the first voltagesoutput by each charge sensing unit sequentially with the ramping voltageto generate the count values.

For measurement of the low intensity range in which the photodiode'squantum well is not saturated by the residual charge, the controller cancontrol the one or more ADCs to perform a PD ADC measurement operationto measure a quantity of the residual charge stored in photodiode. Toperform the PD ADC measurement operation, the controller can control theswitch between the photodiode and the charge sensing unit to transferthe residual charge out of the photodiode into the charge storage deviceto convert to a second voltage. The capacitance of the charge storagedevice can also be reduced (e.g., by disconnecting the auxiliarycapacitor) to maximize the charge-to-voltage conversion gain, which canreduce the quantization error. The one or more ADCs can compare thesecond voltage with a second ramping voltage to generate a decision. Thecounter can start counting at the starting point of the second rampingvoltage, and the memory can store the count value when the secondramping voltage reaches the second voltage. The stored count value canrepresent the quantity of the residual charge. The PD ADC operation (andthe transfer of the residual charge) can be performed after the exposureperiod. In a case where multiple photodiodes share a charge sensingunit, the controller can the perform the PD ADC operation for eachphotodiode sequentially, in which the controller can control theswitches to transfer the residual charge from one photodiode to thecharge sensing unit to generate the second voltage, and control the ADCto quantize the second voltage, and then repeat the operation for otherphotodiodes. As the residual charge is typically much less susceptibleto dark current in the photodiode, the noise floor of the low lightintensity measurement can be lowered, which can further extend the lowerlimit of the dynamic range.

The controller can control the ADCs to perform one or more of theaforementioned quantization operations for each photodiode based on thesharing arrangements of the one or more charge sensing units and the oneor more ADCs among the plurality of the photodiodes, as well as theselection of the quantization operations. As described above, a singlecharge sensing unit can be shared among the plurality of photodiodes. Insome examples, the controller can control the switches to allow eachphotodiode to take turn to transfer overflow charge to the single chargesensing unit to generate the first voltage, which can then be quantizedby the ADC in a TTS operation and/or a FD ADC operation based on theselection. The charge sensing unit can be reset between the start ofquantization operations for a photodiode. Followed by the completion ofthe quantization of the overflow charge, the controller can control theswitches to allow each photodiode to take turn in transferring residualcharge to the charge sensing unit, followed by a PD ADC operation tomeasure the residual charge. While such arrangements allow eachphotodiode to have the same access to the charge sensing unit and thesame set of quantization operations can be performed on the outputs ofeach photodiode, each photodiode may have different effective exposureperiods for accumulating overflow charge, which can degrade the globalshutter operation.

In some examples, the controller can also allow, based on the selection,a first photodiode to transfer overflow charge to the charge sensingunit to perform the TTS and FD ADC operations within the exposureperiod. After the exposure period ends, the controller can control theswitches to allow all of the photodiodes to take turn to transferresidual charge to the charge sensing unit and to perform the PD ADCoperations. Such arrangements can be used when, for example, theintensity of a particular component is very high compared with othercomponents (e.g., in a dark environment with strong infra-redillumination for 3D sensing). The same exposure period can be providedfor each photodiode to either accumulate charge for the strong infra-redcomponent or for the other much weaker visible light components. The TTSand FD ADC operations can be performed on the output of the photodiodethat detects the strong infra-red component, while PD ADC operations canbe performed on the outputs of other photodiodes, which can improve thedynamic range of both the low intensity (for visible light) and the highintensity (for infra-red light) measurement operations.

In some examples, the controller can also some or all of the photodiodesto transfer overflow charge to the charge sensing unit simultaneously.For example, photodiodes configured to detect the same wavelength rangecan transfer overflow charge to the charge sensing unit simultaneously.Moreover, for photodiodes that detect different wavelength ranges, thecontroller can set the biases of their switches to set different quantumwell capacities for these photodiodes. For example, the controller canlower the quantum well capacity for a photodiode associated with aparticular wavelength range which is expected to the be strongest amongother wavelength ranges, such that that photodiode is more likely totransfer overflow charge to the charge sensing unit than otherphotodiodes. Such arrangements not only provide same exposure period foreach photodiode, as in the example described above, but also enhanceflexibility in the light measurement operation. Specifically, while theTTS/FD ADC operation output is more likely to represent the output ofthe expected strongest component of the incident light, when theoperation condition changes and the intensities of other components alsoincrease, the TTS/FD ADC operation output can reflect the other highintensity components of the incident light as well.

In a case where each photodiode has full access to a charge sensingunit, but the charge sensing units share one or more ADCs, thecontroller can also adapt the quantization operations based on thesharing of the ADCs. For example, based on the selection, the controllercan control the switches in the charge sensing units to connect one ofthe photodiode (and its charge sensing unit) to the ADC to perform theTTS operation within the integration period. After the TTS operationcompletes, the controller can control the ADC to perform a FD ADCoperation and/or a PD ADC operation (e.g., based on the selection) foreach of the photodiodes and the corresponding charge sensing unit. Insome examples, the controller can also control the switches in thecharge sensing units to take turn in connecting with the ADC to performthe TTS operation. After the TTS operations for all of the photodiodescomplete, the controller can control the ADC to perform a FD ADCoperation and/or a PD ADC operation for each of the photodiodessequentially.

The memory can be configured to store the quantization results for eachof the photodiodes. In some examples, the memory can be configured tostore the quantization results for all of the photodiodessimultaneously. In some examples, the quantization results of eachphotodiode can be stored in the memory sequentially. The quantizationresults of one photodiode of the pixel cell can be stored in the memory,read out for generation of the pixel value, and then overwritten by thequantization results of another photodiode of the pixel cell.

With examples of the present disclosure, a pixel cell can performcollocated imaging for different components of incident light, which cansupport collocated 2D and 3D imaging operations. Having the same set ofpixel cells to perform sensing of different components can facilitatethe correspondence between images of different components generated bythe pixel cells. Moreover, given that every pixel cell of a pixel cellarray can be used to generate the image, the full spatial resolution ofthe pixel cell array can be utilized. As a result, the spatialresolutions of the images can also be improved. Further, typically theADCs consume a lot of power and occupies a lot of space. By sharing theADCs among the photodiodes of each pixel cell, the form factor and powerconsumption of the image sensor can also be reduced, while the ADCs ofeach pixel cell of a pixel cell array can generate a pixel value basedon incident light received by each pixel cell within the same globalexposure period to support a global shutter operation to reduce imagedistortion.

The disclosed techniques may include or be implemented in conjunctionwith an artificial reality system. Artificial reality is a form ofreality that has been adjusted in some manner before presentation to auser, which may include, e.g., a virtual reality (VR), an augmentedreality (AR), a mixed reality (MR), a hybrid reality, or somecombination and/or derivatives thereof. Artificial reality content mayinclude completely generated content or generated content combined withcaptured (e.g., real-world) content. The artificial reality content mayinclude video, audio, haptic feedback, or some combination thereof, anyof which may be presented in a single channel or in multiple channels(such as stereo video that produces a three-dimensional effect to theviewer). Additionally, in some examples, artificial reality may also beassociated with applications, products, accessories, services, or somecombination thereof, that are used to, e.g., create content in anartificial reality and/or are otherwise used in (e.g., performactivities in) an artificial reality. The artificial reality system thatprovides the artificial reality content may be implemented on variousplatforms, including a head-mounted display (HMD) connected to a hostcomputer system, a standalone HMD, a mobile device or computing system,or any other hardware platform capable of providing artificial realitycontent to one or more viewers.

FIG. 1A is a diagram of an example of a near-eye display 100. Near-eyedisplay 100 presents media to a user. Examples of media presented bynear-eye display 100 include one or more images, video, and/or audio. Insome examples, audio is presented via an external device (e.g., speakersand/or headphones) that receives audio information from the near-eyedisplay 100, a console, or both, and presents audio data based on theaudio information. Near-eye display 100 is generally configured tooperate as a virtual reality (VR) display. In some examples, near-eyedisplay 100 is modified to operate as an augmented reality (AR) displayand/or a mixed reality (MR) display.

Near-eye display 100 includes a frame 105 and a display 110. Frame 105is coupled to one or more optical elements. Display 110 is configuredfor the user to see content presented by near-eye display 100. In someexamples, display 110 comprises a waveguide display assembly fordirecting light from one or more images to an eye of the user.

Near-eye display 100 further includes image sensors 120 a, 120 b, 120 c,and 120 d. Each of image sensors 120 a, 120 b, 120 c, and 120 d mayinclude a pixel cell array configured to generate image datarepresenting different fields of views along different directions. Forexample, sensors 120 a and 120 b may be configured to provide image datarepresenting two fields of view towards a direction A along the Z axis,whereas sensor 120 c may be configured to provide image datarepresenting a field of view towards a direction B along the X axis, andsensor 120 d may be configured to provide image data representing afield of view towards a direction C along the X axis.

In some examples, sensors 120 a-120 d can be configured as input devicesto control or influence the display content of the near-eye display 100,to provide an interactive VR/AR/MR experience to a user who wearsnear-eye display 100. For example, sensors 120 a-120 d can generatephysical image data of a physical environment in which the user islocated. The physical image data can be provided to a location trackingsystem to track a location and/or a path of movement of the user in thephysical environment. A system can then update the image data providedto display 110 based on, for example, the location and orientation ofthe user, to provide the interactive experience. In some examples, thelocation tracking system may operate a SLAM algorithm to track a set ofobjects in the physical environment and within a view of field of theuser as the user moves within the physical environment. The locationtracking system can construct and update a map of the physicalenvironment based on the set of objects, and track the location of theuser within the map. By providing image data corresponding to multiplefields of views, sensors 120 a-120 d can provide the location trackingsystem a more holistic view of the physical environment, which can leadto more objects to be included in the construction and updating of themap. With such an arrangement, the accuracy and robustness of tracking alocation of the user within the physical environment can be improved.

In some examples, near-eye display 100 may further include one or moreactive illuminators 130 to project light into the physical environment.The light projected can be associated with different frequency spectrums(e.g., visible light, infra-red light, ultra-violet light, etc.), andcan serve various purposes. For example, illuminator 130 may projectlight in a dark environment (or in an environment with low intensity ofinfra-red light, ultra-violet light, etc.) to assist sensors 120 a-120 din capturing images of different objects within the dark environment to,for example, enable location tracking of the user. Illuminator 130 mayproject certain markers onto the objects within the environment, toassist the location tracking system in identifying the objects for mapconstruction/updating.

In some examples, illuminator 130 may also enable stereoscopic imaging.For example, one or more of sensors 120 a or 120 b can include both afirst pixel cell array for visible light sensing and a second pixel cellarray for infra-red (IR) light sensing. The first pixel cell array canbe overlaid with a color filter (e.g., a Bayer filter), with each pixelof the first pixel cell array being configured to measure intensity oflight associated with a particular color (e.g., one of red, green orblue colors). The second pixel cell array (for IR light sensing) canalso be overlaid with a filter that allows only IR light through, witheach pixel of the second pixel cell array being configured to measureintensity of IR lights. The pixel cell arrays can generate an RGB imageand an IR image of an object, with each pixel of the IR image beingmapped to each pixel of the RGB image. Illuminator 130 may project a setof IR markers on the object, the images of which can be captured by theIR pixel cell array. Based on a distribution of the IR markers of theobject as shown in the image, the system can estimate a distance ofdifferent parts of the object from the IR pixel cell array, and generatea stereoscopic image of the object based on the distances. Based on thestereoscopic image of the object, the system can determine, for example,a relative position of the object with respect to the user, and canupdate the image data provided to display 100 based on the relativeposition information to provide the interactive experience.

As discussed above, near-eye display 100 may be operated in environmentsassociated with a very wide range of light intensities. For example,near-eye display 100 may be operated in an indoor environment or in anoutdoor environment, and/or at different times of the day. Near-eyedisplay 100 may also operate with or without active illuminator 130being turned on. As a result, image sensors 120 a-120 d may need to havea wide dynamic range to be able to operate properly (e.g., to generatean output that correlates with the intensity of incident light) across avery wide range of light intensities associated with different operatingenvironments for near-eye display 100.

FIG. 1B is a diagram of another example of near-eye display 100. FIG. 1Billustrates a side of near-eye display 100 that faces the eyeball(s) 135of the user who wears near-eye display 100. As shown in FIG. 1B,near-eye display 100 may further include a plurality of illuminators 140a, 140 b, 140 c, 140 d, 140 e, and 140 f. Near-eye display 100 furtherincludes a plurality of image sensors 150 a and 150 b. Illuminators 140a, 140 b, and 140 c may emit lights of certain frequency range (e.g.,NIR) towards direction D (which is opposite to direction A of FIG. 1A).The emitted light may be associated with a certain pattern, and can bereflected by the left eyeball of the user. Sensor 150 a may include apixel cell array to receive the reflected light and generate an image ofthe reflected pattern. Similarly, illuminators 140 d, 140 e, and 140 fmay emit NIR lights carrying the pattern. The NIR lights can bereflected by the right eyeball of the user, and may be received bysensor 150 b. Sensor 150 b may also include a pixel cell array togenerate an image of the reflected pattern. Based on the images of thereflected pattern from sensors 150 a and 150 b, the system can determinea gaze point of the user, and update the image data provided to display100 based on the determined gaze point to provide an interactiveexperience to the user.

As discussed above, to avoid damaging the eyeballs of the user,illuminators 140 a, 140 b, 140 c, 140 d, 140 e, and 140 f are typicallyconfigured to output lights of very low intensities. In a case whereimage sensors 150 a and 150 b comprise the same sensor devices as imagesensors 120 a-120 d of FIG. 1A, the image sensors 120 a-120 d may needto be able to generate an output that correlates with the intensity ofincident light when the intensity of the incident light is very low,which may further increase the dynamic range requirement of the imagesensors.

Moreover, the image sensors 120 a-120 d may need to be able to generatean output at a high speed to track the movements of the eyeballs. Forexample, a user's eyeball can perform a very rapid movement (e.g., asaccade movement) in which there can be a quick jump from one eyeballposition to another. To track the rapid movement of the user's eyeball,image sensors 120 a-120 d need to generate images of the eyeball at highspeed. For example, the rate at which the image sensors generate animage frame (the frame rate) needs to at least match the speed ofmovement of the eyeball. The high frame rate requires short totalexposure time for all of the pixel cells involved in generating theimage frame, as well as high speed for converting the sensor outputsinto digital values for image generation. Moreover, as discussed above,the image sensors also need to be able to operate at an environment withlow light intensity.

FIG. 2 is an example of a cross section 200 of near-eye display 100illustrated in FIG. 1 . Display 110 includes at least one waveguidedisplay assembly 210. An exit pupil 230 is a location where a singleeyeball 220 of the user is positioned in an eyebox region when the userwears the near-eye display 100. For purposes of illustration, FIG. 2shows the cross section 200 associated eyeball 220 and a singlewaveguide display assembly 210, but a second waveguide display is usedfor a second eye of a user.

Waveguide display assembly 210 is configured to direct image light to aneyebox located at exit pupil 230 and to eyeball 220. Waveguide displayassembly 210 may be composed of one or more materials (e.g., plastic,glass, etc.) with one or more refractive indices. In some examples,near-eye display 100 includes one or more optical elements betweenwaveguide display assembly 210 and eyeball 220.

In some examples, waveguide display assembly 210 includes a stack of oneor more waveguide displays including, but not restricted to, a stackedwaveguide display, a varifocal waveguide display, etc. The stackedwaveguide display is a polychromatic display (e.g., a red-green-blue(RGB) display) created by stacking waveguide displays whose respectivemonochromatic sources are of different colors. The stacked waveguidedisplay is also a polychromatic display that can be projected onmultiple planes (e.g., multi-planar colored display). In someconfigurations, the stacked waveguide display is a monochromatic displaythat can be projected on multiple planes (e.g., multi-planarmonochromatic display). The varifocal waveguide display is a displaythat can adjust a focal position of image light emitted from thewaveguide display. In alternate examples, waveguide display assembly 210may include the stacked waveguide display and the varifocal waveguidedisplay.

FIG. 3 illustrates an isometric view of an example of a waveguidedisplay 300. In some examples, waveguide display 300 is a component(e.g., waveguide display assembly 210) of near-eye display 100. In someexamples, waveguide display 300 is part of some other near-eye displayor other system that directs image light to a particular location.

Waveguide display 300 includes a source assembly 310, an outputwaveguide 320, and a controller 330. For purposes of illustration, FIG.3 shows the waveguide display 300 associated with a single eyeball 220,but in some examples, another waveguide display separate, or partiallyseparate, from the waveguide display 300 provides image light to anothereye of the user.

Source assembly 310 generates image light 355. Source assembly 310generates and outputs image light 355 to a coupling element 350 locatedon a first side 370-1 of output waveguide 320. Output waveguide 320 isan optical waveguide that outputs expanded image light 340 to an eyeball220 of a user. Output waveguide 320 receives image light 355 at one ormore coupling elements 350 located on the first side 370-1 and guidesreceived input image light 355 to a directing element 360. In someexamples, coupling element 350 couples the image light 355 from sourceassembly 310 into output waveguide 320. Coupling element 350 may be,e.g., a diffraction grating, a holographic grating, one or more cascadedreflectors, one or more prismatic surface elements, and/or an array ofholographic reflectors.

Directing element 360 redirects the received input image light 355 todecoupling element 365 such that the received input image light 355 isdecoupled out of output waveguide 320 via decoupling element 365.Directing element 360 is part of, or affixed to, first side 370-1 ofoutput waveguide 320. Decoupling element 365 is part of, or affixed to,second side 370-2 of output waveguide 320, such that directing element360 is opposed to the decoupling element 365. Directing element 360and/or decoupling element 365 may be, e.g., a diffraction grating, aholographic grating, one or more cascaded reflectors, one or moreprismatic surface elements, and/or an array of holographic reflectors.

Second side 370-2 represents a plane along an x-dimension and ay-dimension. Output waveguide 320 may be composed of one or morematerials that facilitate total internal reflection of image light 355.Output waveguide 320 may be composed of e.g., silicon, plastic, glass,and/or polymers. Output waveguide 320 has a relatively small formfactor. For example, output waveguide 320 may be approximately 50 mmwide along x-dimension, 30 mm long along y-dimension and 0.5-1 mm thickalong a z-dimension.

Controller 330 controls scanning operations of source assembly 310. Thecontroller 330 determines scanning instructions for the source assembly310. In some examples, the output waveguide 320 outputs expanded imagelight 340 to the user's eyeball 220 with a large field of view (FOV).For example, the expanded image light 340 is provided to the user'seyeball 220 with a diagonal FOV (in x and y) of 60 degrees and/orgreater and/or 150 degrees and/or less. The output waveguide 320 isconfigured to provide an eyebox with a length of 20 mm or greater and/orequal to or less than 50 mm; and/or a width of 10 mm or greater and/orequal to or less than 50 mm.

Moreover, controller 330 also controls image light 355 generated bysource assembly 310, based on image data provided by image sensor 370.Image sensor 370 may be located on first side 370-1 and may include, forexample, image sensors 120 a-120 d of FIG. 1A to generate image data ofa physical environment in front of the user (e.g., for locationdetermination). Image sensor 370 may also be located on second side370-2 and may include image sensors 150 a and 150 b of FIG. 1B togenerate image data of eyeball 220 (e.g., for gaze point determination)of the user. Image sensor 370 may interface with a remote console thatis not located within waveguide display 300. Image sensor 370 mayprovide image data to the remote console, which may determine, forexample, a location of the user, a gaze point of the user, etc., anddetermine the content of the images to be displayed to the user. Theremote console can transmit instructions to controller 330 related tothe determined content. Based on the instructions, controller 330 cancontrol the generation and outputting of image light 355 by sourceassembly 310.

FIG. 4 illustrates an example of a cross section 400 of the waveguidedisplay 300. The cross section 400 includes source assembly 310, outputwaveguide 320, and image sensor 370. In the example of FIG. 4 , imagesensor 370 may include a set of pixel cells 402 located on first side370-1 to generate an image of the physical environment in front of theuser. In some examples, there can be a mechanical shutter 404 interposedbetween the set of pixel cells 402 and the physical environment tocontrol the exposure of the set of pixel cells 402. In some examples,the mechanical shutter 404 can be replaced by an electronic shuttergate, as to be discussed below. Each of pixel cells 402 may correspondto one pixel of the image. Although not shown in FIG. 4 , it isunderstood that each of pixel cells 402 may also be overlaid with afilter to control the frequency range of the light to be sensed by thepixel cells.

After receiving instructions from the remote console, mechanical shutter404 can open and expose the set of pixel cells 402 in an exposureperiod. During the exposure period, image sensor 370 can obtain samplesof lights incident on the set of pixel cells 402, and generate imagedata based on an intensity distribution of the incident light samplesdetected by the set of pixel cells 402. Image sensor 370 can thenprovide the image data to the remote console, which determines thedisplay content, and provide the display content information tocontroller 330. Controller 330 can then determine image light 355 basedon the display content information.

Source assembly 310 generates image light 355 in accordance withinstructions from the controller 330. Source assembly 310 includes asource 410 and an optics system 415. Source 410 is a light source thatgenerates coherent or partially coherent light. Source 410 may be, e.g.,a laser diode, a vertical cavity surface emitting laser, and/or a lightemitting diode.

Optics system 415 includes one or more optical components that conditionthe light from source 410. Conditioning light from source 410 mayinclude, e.g., expanding, collimating, and/or adjusting orientation inaccordance with instructions from controller 330. The one or moreoptical components may include one or more lenses, liquid lenses,mirrors, apertures, and/or gratings. In some examples, optics system 415includes a liquid lens with a plurality of electrodes that allowsscanning of a beam of light with a threshold value of scanning angle toshift the beam of light to a region outside the liquid lens. Lightemitted from the optics system 415 (and also source assembly 310) isreferred to as image light 355.

Output waveguide 320 receives image light 355. Coupling element 350couples image light 355 from source assembly 310 into output waveguide320. In examples where coupling element 350 is diffraction grating, apitch of the diffraction grating is chosen such that total internalreflection occurs in output waveguide 320, and image light 355propagates internally in output waveguide 320 (e.g., by total internalreflection), toward decoupling element 365.

Directing element 360 redirects image light 355 toward decouplingelement 365 for decoupling from output waveguide 320. In examples wheredirecting element 360 is a diffraction grating, the pitch of thediffraction grating is chosen to cause incident image light 355 to exitoutput waveguide 320 at angle(s) of inclination relative to a surface ofdecoupling element 365.

In some examples, directing element 360 and/or decoupling element 365are structurally similar. Expanded image light 340 exiting outputwaveguide 320 is expanded along one or more dimensions (e.g., may beelongated along x-dimension). In some examples, waveguide display 300includes a plurality of source assemblies 310 and a plurality of outputwaveguides 320. Each of source assemblies 310 emits a monochromaticimage light of a specific band of wavelength corresponding to a primarycolor (e.g., red, green, or blue). Each of output waveguides 320 may bestacked together with a distance of separation to output an expandedimage light 340 that is multi-colored.

FIG. 5 is a block diagram of an example of a system 500 including thenear-eye display 100. The system 500 comprises near-eye display 100, animaging device 535, an input/output interface 540, and image sensors 120a-120 d and 150 a-150 b that are each coupled to control circuitries510. System 500 can be configured as a head-mounted device, a wearabledevice, etc.

Near-eye display 100 is a display that presents media to a user.Examples of media presented by the near-eye display 100 include one ormore images, video, and/or audio. In some examples, audio is presentedvia an external device (e.g., speakers and/or headphones) that receivesaudio information from near-eye display 100 and/or control circuitries510 and presents audio data based on the audio information to a user. Insome examples, near-eye display 100 may also act as an AR eyewear glass.In some examples, near-eye display 100 augments views of a physical,real-world environment, with computer-generated elements (e.g., images,video, sound, etc.).

Near-eye display 100 includes waveguide display assembly 210, one ormore position sensors 525, and/or an inertial measurement unit (IMU)530. Waveguide display assembly 210 includes source assembly 310, outputwaveguide 320, and controller 330.

IMU 530 is an electronic device that generates fast calibration dataindicating an estimated position of near-eye display 100 relative to aninitial position of near-eye display 100 based on measurement signalsreceived from one or more of position sensors 525.

Imaging device 535 may generate image data for various applications. Forexample, imaging device 535 may generate image data to provide slowcalibration data in accordance with calibration parameters received fromcontrol circuitries 510. Imaging device 535 may include, for example,image sensors 120 a-120 d of FIG. 1A for generating image data of aphysical environment in which the user is located, for performinglocation tracking of the user. Imaging device 535 may further include,for example, image sensors 150 a-150 b of FIG. 1B for generating imagedata for determining a gaze point of the user, to identify an object ofinterest of the user.

The input/output interface 540 is a device that allows a user to sendaction requests to the control circuitries 510. An action request is arequest to perform a particular action. For example, an action requestmay be to start or end an application or to perform a particular actionwithin the application.

Control circuitries 510 provide media to near-eye display 100 forpresentation to the user in accordance with information received fromone or more of: imaging device 535, near-eye display 100, andinput/output interface 540. In some examples, control circuitries 510can be housed within system 500 configured as a head-mounted device. Insome examples, control circuitries 510 can be a standalone consoledevice communicatively coupled with other components of system 500. Inthe example shown in FIG. 5 , control circuitries 510 include anapplication store 545, a tracking module 550, and an engine 555.

The application store 545 stores one or more applications for executionby the control circuitries 510. An application is a group ofinstructions, that, when executed by a processor, generates content forpresentation to the user. Examples of applications include: gamingapplications, conferencing applications, video playback applications, orother suitable applications.

Tracking module 550 calibrates system 500 using one or more calibrationparameters and may adjust one or more calibration parameters to reduceerror in determination of the position of the near-eye display 100.

Tracking module 550 tracks movements of near-eye display 100 using slowcalibration information from the imaging device 535. Tracking module 550also determines positions of a reference point of near-eye display 100using position information from the fast calibration information.

Engine 555 executes applications within system 500 and receives positioninformation, acceleration information, velocity information, and/orpredicted future positions of near-eye display 100 from tracking module550. In some examples, information received by engine 555 may be usedfor producing a signal (e.g., display instructions) to waveguide displayassembly 210 that determines a type of content presented to the user.For example, to provide an interactive experience, engine 555 maydetermine the content to be presented to the user based on a location ofthe user (e.g., provided by tracking module 550), or a gaze point of theuser (e.g., based on image data provided by imaging device 535), adistance between an object and user (e.g., based on image data providedby imaging device 535).

FIG. 6 illustrates an example of an image sensor 600. Image sensor 600can be part of near-eye display 100, and can provide 2D and 3D imagedata to control circuitries 510 of FIG. 5 to control the display contentof near-eye display 100. As shown in FIG. 6 , image sensor 600 mayinclude an array of pixel cells 602 including pixel cell 602 a. Pixelcell 602 a can include a plurality of photodiodes 612 including, forexample, photodiodes 612 a, 612 b, 612 c, and 612 d, one or more chargesensing units 614, and one or more analog-to-digital converters 616. Theplurality of photodiodes 612 can convert different components ofincident light to charge. For example, photodiode 612 a-612 c cancorrespond to different visible light channels, in which photodiode 612a can convert a visible blue component (e.g., a wavelength range of450-490 nanometers (nm)) to charge. Photodiode 612 b can convert avisible green component (e.g., a wavelength range of 520-560 nm) tocharge. Photodiode 612 c can convert a visible red component (e.g., awavelength range of 635-700 nm) to charge. Moreover, photodiode 612 dcan convert an infra-red component (e.g., 700-1000 nm) to charge. Eachof the one or more charge sensing units 614 can include a charge storagedevice and a buffer to convert the charge generated by photodiodes 612a-612 d to voltages, which can be quantized by one or more ADCs 616 intodigital values. The digital values generated from photodiodes 612 a-612c can represent the different visible light components of a pixel, andeach can be used for 2D sensing in a particular visible light channel.Moreover, the digital value generated from photodiode 612 d canrepresent the infra-red light component of the same pixel and can beused for 3D sensing. Although FIG. 6 shows that pixel cell 602 aincludes four photodiodes, it is understood that the pixel cell caninclude a different number of photodiodes (e.g., two, three, etc.).

In addition, image sensor 600 also includes an illuminator 622, anoptical filter 624, an imaging module 628, and a sensing controller 630.Illuminator 622 may be an infra-red illuminator, such as a laser, alight emitting diode (LED), etc., that can project infra-red light for3D sensing. The projected light may include, for example, structuredlight, light pulses, etc. Optical filter 624 may include an array offilter elements overlaid on the plurality of photodiodes 612 a-612 d ofeach pixel cell including pixel cell 606 a. Each filter element can seta wavelength range of incident light received by each photodiode ofpixel cell 606 a. For example, a filter element over photodiode 612 amay transmit the visible blue light component while blocking othercomponents, a filter element over photodiode 612 b may transmit thevisible green light component, a filter element over photodiode 612 cmay transmit the visible red light component, whereas a filter elementover photodiode 612 d may transmit the infra-red light component.

Image sensor 600 further includes an imaging module 628. Imaging module628 may further include a 2D imaging module 632 to perform 2D imagingoperations and a 3D imaging module 634 to perform 3D imaging operations.The operations can be based on digital values provided by ADCs 616. Forexample, based on the digital values from each of photodiodes 612 a-612c, 2D imaging module 632 can generate an array of pixel valuesrepresenting an intensity of an incident light component for eachvisible color channel, and generate an image frame for each visiblecolor channel. Moreover, 3D imaging module 634 can generate a 3D imagebased on the digital values from photodiode 612 d. In some examples,based on the digital values, 3D imaging module 634 can detect a patternof structured light reflected by a surface of an object, and compare thedetected pattern with the pattern of structured light projected byilluminator 622 to determine the depths of different points of thesurface with respect to the pixel cells array. For detection of thepattern of reflected light, 3D imaging module 634 can generate pixelvalues based on intensities of infra-red light received at the pixelcells. As another example, 3D imaging module 634 can generate pixelvalues based on time-of-flight of the infra-red light transmitted byilluminator 622 and reflected by the object.

Image sensor 600 further includes a sensing controller 640 to controldifferent components of image sensor 600 to perform 2D and 3D imaging ofan object. Reference is now made to FIG. 7A-FIG. 7C, which illustrateexamples of operations of image sensor 600 for 2D and 3D imaging. FIG.7A illustrates an example of operations for 2D imaging. For 2D imaging,pixel cells array 602 can detect visible light in the environmentincluding visible light reflected off an object. For example, referringto FIG. 7A, visible light source 700 (e.g., a light bulb, the sun, orother sources of ambient visible light) can project visible light 702onto an object 704. Visible light 706 can be reflected off a spot 708 ofobject 704. Visible light 706 can also include the ambient infra-redlight component. Visible light 706 can be filtered by optical filterarray 624 to pass different components of visible light 706 ofwavelength ranges w0, w1, w2, and w3 to, respectively, photodiodes 612a, 612 b, 612 c, and 612 d of pixel cell 602 a. Wavelength ranges w0,w1, w2, and w3 an correspond to, respectively, blue, green, red, andinfra-red. As shown in FIG. 7A, as the infra-red illuminator 622 is notturned on, the intensity of infra-red component (w3) is contributed bythe ambient infra-red light and can be very low. Moreover, differentvisible components of visible light 706 can also have differentintensities. Charge sensing units 614 can convert the charge generatedby the photodiodes to voltages, which can be quantized by ADCs 616 intodigital values representing the red, blue, and green components of apixel representing spot 708. Referring to FIG. 7C, after the digitalvalues are generated, sensing controller 640 can control 2D imagingmodule 632 to generate, based on the digital values, sets of imagesincluding a set of images 710, which includes a red image frame 710 a, ablue image frame 710 b, and a green image frame 710 c each representingone of red, blue, or green color image of a scene captured with the sameexposure period 714. Each pixel from the red image (e.g., pixel 712 a),from the blue image (e.g., pixel 712 b), and from the green image (e.g.,pixel 712 c) can represent visible components of light from the samespot (e.g., spot 708) of a scene. A different set of images 720 can begenerated by 2D imaging module 632 in a subsequent exposure period 724.Each of red image 710 a, blue image 710 b, and green image 710 c canrepresent the scene in a specific color channel and can be provided toan application to, for example, extract image features from the specificcolor channel. As each image represents the same scene and eachcorresponding pixel of the images represent light from the same spot ofthe scene, the correspondence of images between different color channelscan be improved.

Furthermore, image sensor 600 can also perform 3D imaging of object 704.Referring to FIG. 7B, sensing controller 610 can control illuminator 622to project infra-red light 732, which can include a light pulse,structured light, etc., onto object 704. Infra-red light 732 can have awavelength range of 700 nanometers (nm) to 1 millimeter (mm). Infra-redlight 734 can reflect off spot 708 of object 704 and can propagatetowards pixel cells array 602 and pass through optical filter 624, whichcan provide the infra-red component (of wavelength range w3) tophotodiode 612 d to convert to charge. Charge sensing units 614 canconvert the charge to a voltage, which can be quantized by ADCs 616 intodigital values. Referring to FIG. 7C, after the digital values aregenerated, sensing controller 640 can control 3D imaging module 634 togenerate, based on the digital values, an infra-red image 710 d of thescene as part of images 710 captured within exposure period 714. Asinfra-red image 710 d can represent the same scene in the infra-redchannel and a pixel of infra-red image 710 d (e.g., pixel 712 d)represents light from the same spot of the scene as other correspondingpixels (pixels 712 a-712 c) in other images within images 710, thecorrespondence between 2D and 3D imaging can be improved as well.

FIG. 8A-FIG. 8D illustrate examples of arrangements of photodiodes 612in a pixel cell. As shown in FIG. 8A, the photodiodes 612 a-612 d in apixel cell 602 a can form a stack along an axis that is perpendicular toa light receiving surface 800 through which pixel cell 602 a receivesincident light 802 from a spot 804 a. For example, the photodiodes 612a-612 d can form a stack along a vertical axis (e.g., the z-axis) whenthe light receiving surface 800 is parallel with the x and y axes. Eachphotodiode can have a different distance from light receiving surface800, and the distance can set the component of incident light 802 beingabsorbed and converted to charge by each photodiode. For example,photodiode 612 a is closest to light receiving surface 800 and canabsorb and convert the blue component to charge, which is of theshortest wavelength range among the other components. Light 812 includesthe remaining components of light 802 (e.g., green, red, and infra-red)and can propagate to photodiode 612 b, which can absorb and convert thegreen component. Light 822 includes the remaining components of light812 (e.g., red and infra-red) and can propagate to photodiode 612 c,which can absorb and convert the red component. The remaining infra-redcomponent 832 can propagate to photodiode 612 d to be converted tocharge.

Each the photodiodes 612 a, 612 b, 612 c, and 612 d can be in a separatesemiconductor substrate, which can be stacked to form image sensor 600.For example, photodiode 612 a can be in a semiconductor substrate 840,photodiode 612 b can be in a semiconductor substrate 842, photodiode 612c can be in a semiconductor substrate 844, whereas photodiode 612 d canbe in a semiconductor substrate 846. Each semiconductor substrate caninclude other photodiodes of other pixel cells, such as pixel cells 602b to receive light from spot 804 b. Image sensor 600 can include anothersemiconductor substrate 848 which can include pixel cell processingcircuits 849 which can include, for example, charge sensing units 614,ADCs 616, etc. Each semiconductor substrate can be connected to a metalinterconnect, such as metal interconnects 850, 852, 854, and 856 totransfer the charge generated at each photodiode to processing circuit849.

FIG. 8B-FIG. 8D illustrate other example arrangements of photodiodes612. As shown in FIG. 8B-FIG. 8D, the plurality of photodiodes 612 canbe arranged laterally parallel with light receiving surface 800. The topgraph of FIG. 8B illustrates a side view of an example of a pixel cellarray 602 including pixel cell 602 a, whereas the bottom graph of FIG.8B illustrates a top view of pixel cell array 602 including pixel cell602 a. As shown in FIG. 8B, with light receiving surface 800 beingparallel with the x and y axes, photodiodes 612 a, 612 b, 612 c, and 612d can be arranged adjacent to each other also along the x and y axes insemiconductor substrate 840. Pixel cell 602 a further includes anoptical filter array 860 overlaid on the photodiodes. Optical filterarray 860 can be part of optical filter 624. Optical filter array 860can include a filter element overlaid on each of photodiodes 612 a, 612b, 612 c, and 612 d to set a wavelength range of incident lightcomponent received by the respective photodiode. For example, filterelement 860 a is overlaid on photodiode 612 a and can allow only visibleblue light to enter photodiode 612 a. Moreover, filter element 860 b isoverlaid on photodiode 612 b and can allow only visible green light toenter photodiode 612 b. Further, filter element 860 c is overlaid onphotodiode 612 c and can allow only visible red light to enterphotodiode 612 c. Filter element 860 d is overlaid on photodiode 612 dand can allow only infra-red light to enter photodiode 612 d. Pixel cell602 a further includes one or more microlens 862 which can project light864 from a spot of a scene (e.g., spot 804 a) via optical tiler array860 to different lateral locations of light receiving surface 800, whichallows each photodiode to become a sub-pixel of pixel cell 602 a and toreceive components of light from the same spot corresponding to a pixel.In some examples, the one or more microlens 862 can include a microlenscovering all pixel cells of pixel cell array 602. In some examples, theone or more microlens 862 may include a microlens over each pixel cell,and multiple microlens cover multiple pixel cells. Pixel cell 602 a canalso include semiconductor substrate 848 which can include circuit 849(e.g., charge sensing units 614, ADCs 616, etc.) to generate digitalvalues from the charge generated by the photodiodes. Semiconductorsubstrates 840 and 848 can form a stack and can be connected withinterconnect 856.

The arrangements of FIG. 8B, in which the photodiodes are arrangedlaterally and an optical filter array is used to control the lightcomponents received by the photodiodes, can offer numerous advantages.For example, the number of stacks and the number of semiconductorsubstrates can be reduced, which not only reduce the vertical height butalso the interconnects among the semiconductor substrates. Moreover,relying on filter elements rather than the propagation distance of lightto set the wavelength ranges of the components absorbed by eachphotodiode can offer flexibilities in selecting the wavelength ranges.As shown in top graph of FIG. 8C, pixel cells array 602 can includedifferent optical filter arrays 860 for different pixel cells. Forexample, each pixel cell of pixel cells array 602 can have an opticalfilter array that provides monochrome channel of a wavelength range of380-740 nm (labelled with “M”) for photodiodes 612 a and 612 b, and aninfra-red channel of a wavelength range of 700-1000 nm (labelled with“NIR”) for photodiode 612 d. But the optical filter arrays may alsoprovide a different visible color channel for the different pixel cells.For example, the optical filter arrays 860 for pixel cells array 602 a,602 b, 602 c, and 602 d may provide, respectively, a visible greenchannel (labelled with “G”), a visible red channel (labelled with “R”),a visible blue channel (labelled with “B”), and a visible green channelfor photodiode 612 c of the pixel cells arrays. As another example, asshown in the bottom graph of FIG. 8C, each optical filter array 860 canprovide a monochrome and infra-red channel (labelled “M+NIR”) whichspans a wavelength range of 380-1000 nm for photodiode 612 b of eachpixel cells array.

FIG. 8D illustrates examples of optical filter array 860 to provide theexample channels shown in FIG. 8C. As shown in FIG. 8D, optical filterarray 860 can include a stack of optical filters to select a wavelengthrange of light received by each photodiode within a pixel cell array.For example, referring to the top graph of FIG. 8D, optical filter 860 acan include an all-pass element 870 (e.g., a transparent glass thatpasses both visible light and infra-red light) and an infra-red blockingelement 872 forming a stack to provide a monochrome channel forphotodiode 612 a. Optical filter 860 b can also include an all-passelement 874 and an infra-red blocking element 876 to also provide amonochrome channel for photodiode 612 b. Further, optical filter 860 ccan include a green-pass element 876 which passes green visible light(but reject other visible light component), and an infra-red blockingelement 878, to provide a green channel for photodiode 612 c. Lastly,optical filter 860 d can include an all-pass element 880 and a visiblelight blocking filter 882 (which can block out visible light but allowsinfra-red light to go through) to provide an infra-red channel forphotodiode 612 d. In another example, as shown in the bottom graph ofFIG. 8D, optical filter 860 b can include only all-pass element 872 toprovide a monochrome and infra-red channel for photodiode 612 b.

Reference is now made to FIGS. 9A-9F, which illustrates additionalcomponents of pixel cell 602 a including an example of charge sensingunit 614 and ADC 616. As shown in FIG. 9A, pixel cell 602 a can includea photodiode PD (e.g., photodiode 612 a), a shutter switch M0, atransfer switch M1, a charge sensing unit 614 comprising a chargestorage device 902 and a switchable buffer 904, and an ADC 616comprising a CC capacitor, a comparator 906, and output logic circuits908. The output of comparator 906 is coupled, via output logic circuits908, with a memory 912 and a counter 914 which can be internal to orexternal to pixel cell 602 a. Pixel cell 602 further includes acontroller 920 to control the switches, charge sensing unit 614, as wellas ADC 616, As to be described below, controller 920 can set an exposureperiod to accumulate charge based on incident light, and can controlcharge sensing unit 614 and ADC 616 to perform multiple quantizationoperations associated with different light intensity ranges to generatea digital representation of the intensity of the incident light.Controller 920 can receive a selection 922 to selection which of themultiple quantization operations to be performed (and which is to beskipped). The selection can come from a host device which hosts anapplication that uses the digital representation of incident lightintensity. Controller 920 can be internal to pixel cell 602 a or part ofsensing controller 640. Each switch can be a transistor such as, forexample, a metal-oxide-semiconductor field-effect transistor (MOSFET), abipolar junction transistor (BJT), etc.

Specifically, shutter switch M0 can be controlled by an AB signalprovided by controller 920 to start an exposure period, in which thephotodiode PD can generate and accumulate charge in response to incidentlight. Transfer switch M1 can be controlled by a TG signal provided bycontroller 920 to transfer some of the charge to charge storage device902. In one quantization operation, transfer switch M1 can be biased ata partially-on state to set a quantum well capacity of photodiode PD,which also sets a quantity of residual charge stored at the photodiodePD. After the photodiode PD is saturated by the residual charge,overflow charge can flow through transfer switch M1 to charge storagedevice 902. In another quantization operation, transfer switch M1 can befully turned on to transfer the residual charge from the photodiode PDto charge storage device for measurement.

Charge storage device 902 has a configurable capacity and can convertthe charge transferred from switch M1 to a voltage at the OF node.Charge storage device 902 includes a C_(FD) capacitor (e.g., a floatingdrain) and a C_(EXT) capacitor (e.g., an MOS capacitor) connected by aM6 switch. M6 switch can be enabled by a LG signal to expand thecapacity of charge storage device 902 by connecting C_(FD) and C_(EXT)capacitors in parallel, or to reduce the capacity by disconnecting thecapacitors from each other. The capacity of charge storage device 902can be reduced for measurement of residual charge to increase thecharge-to-voltage gain and to reduce the quantization error. Moreover,the capacity of charge storage device 902 can also be increased formeasurement of overflow charge to reduce the likelihood of saturationand to improve non-linearity. As to be described below, the capacity ofcharge storage device 902 can be adjusted for measurement of differentlight intensity ranges. Charge storage device 902 is also coupled with areset switch M2 which can be controlled by a reset signal RST, providedby controller 920, to reset C_(FD) and C_(EXT) capacitors betweendifferent quantization operations.

Switchable buffer 904 can be include a switch M3 configured as a sourcefollower to buffer the voltage at the OF node to improve its drivingstrength. The buffered voltage can be at the input node PIXEL_OUT of ADC616. The M4 transistor provides a current source for switchable buffer904 and can be biased by a VB signal. Switchable buffer 904 alsoincludes a switch M5 which be enabled or disabled by a SEL signal. Whenswitch M5 is disabled, source follower M3 can be disconnected from thePIXEL_OUT node. As to be described below, pixel cell 602 a may includemultiple charge sensing units 614 each including a switchable buffer904, and one of the charge sensing units can be coupled with PIXEL_OUT(and ADC 616) at one time based on the SEL signal.

As described above, charge generated by photodiode PD within an exposureperiod can be temporarily stored in charge storage device 902 andconverted to a voltage. The voltage can be quantized to represent anintensity of the incident light based on a pre-determined relationshipbetween the charge and the incident light intensity. Reference is nowmade to FIG. 10 , which illustrates a quantity of charge accumulatedwith respect to time for different light intensity ranges. The totalquantity of charge accumulated at a particular time point can reflectthe intensity of light incident upon photodiode PD of FIG. 6 within anexposure period. The quantity can be measured when the exposure periodends. A threshold 1002 and a threshold 1004 can be defined for athreshold's quantity of charge defining a low light intensity range1006, a medium light intensity range 1008, and a high light intensityrange 1010 for the intensity of the incident light. For example, if thetotal accumulated charge is below threshold 1002 (e.g., Q1), theincident light intensity is within low light intensity range 1006. Ifthe total accumulated charge is between threshold 1004 and threshold1002 (e.g., Q2), the incident light intensity is within medium lightintensity range 1008. If the total accumulated charge is above threshold1004, the incident light intensity is within medium light intensityrange 1010. The quantity of the accumulated charge, for low and mediumlight intensity ranges, can correlate with the intensity of the incidentlight, if the photodiode does not saturate within the entire low lightintensity range 1006 and the measurement capacitor does not saturatewithin the entire medium light intensity range 1008.

The definitions of low light intensity range 1006 and medium lightintensity range 1008, as well as thresholds 1002 and 1004, can be basedon the full well capacity of photodiode PD and the capacity of chargestorage device 902. For example, low light intensity range 706 can bedefined such that the total quantity of residual charge stored inphotodiode PD, at the end of the exposure period, is below or equal tothe storage capacity of the photodiode, and threshold 1002 can be basedon the full well capacity of photodiode PD. Moreover, medium lightintensity range 1008 can be defined such that the total quantity ofcharge stored in charge storage device 902, at the end of the exposureperiod, is below or equal to the storage capacity of the measurementcapacitor, and threshold 1004 can be based on the storage capacity ofcharge storage device 902. Typically threshold 1004 is can be based on ascaled storage capacity of charge storage device 902 to ensure that whenthe quantity of charge stored in charge storage device 902 is measuredfor intensity determination, the measurement capacitor does notsaturate, and the measured quantity also relates to the incident lightintensity. As to be described below, thresholds 1002 and 1004 can beused to detect whether photodiode PD and charge storage device 902saturate, which can determine the intensity range of the incident light.

In addition, in a case where the incident light intensity is within highlight intensity range 1010, the total overflow charge accumulated atcharge storage device 902 may exceed threshold 1004 before the exposureperiod ends. As additional charge is accumulated, charge storage device902 may reach full capacity before the end of the exposure period, andcharge leakage may occur. To avoid measurement error caused due tocharge storage device 902 reaching full capacity, a time-to-saturationmeasurement can be performed to measure the time duration it takes forthe total overflow charge accumulated at charge storage device 902 toreach threshold 1004. A rate of charge accumulation at charge storagedevice 902 can be determined based on a ratio between threshold 1004 andthe time-to-saturation, and a hypothetical quantity of charge (Q3) thatcould have been accumulated at charge storage device 902 at the end ofthe exposure period (if the capacitor had limitless capacity) can bedetermined by extrapolation according to the rate of chargeaccumulation. The hypothetical quantity of charge (Q3) can provide areasonably accurate representation of the incident light intensitywithin high light intensity range 1010.

Referring back to FIG. 9 , to measure high light intensity range 1010and medium light intensity range 1008, transfer switch M1 can be biasedby TG signal in a partially turned-on state. For example, the gatevoltage of transfer switch M1 (TG) can be set based on a target voltagedeveloped at photodiode PD corresponding to the full well capacity ofthe photodiode. With such arrangements, only overflow charge (e.g.,charge generated by the photodiode after the photodiode saturates) willtransfer through transfer switch M1 to reach charge storage device 902,to measure time-to-saturation (for high light intensity range 1010)and/or the quantity of charge stored in charge storage device 902 (formedium light intensity range 1008). For measurement of medium and highlight intensity ranges, the capacitance of charge storage device 902 (byconnecting C_(EXT) and C_(FD)) can also be maximized to increasethreshold 1004.

Moreover, to measure low light intensity range 1006, transfer switch M1can be controlled in a fully turned-on state to transfer the residualcharge stored in photodiode PD to charge storage device 902. Thetransfer can occur after the quantization operation of the overflowcharge stored at charge storage device 902 completes and after chargestorage device 902 is reset. Moreover, the capacitance of charge storagedevice 902 can be reduced. As described above, the reduction in thecapacitance of charge storage device 902 can increase thecharge-to-voltage conversion ratio at charge storage device 902, suchthat a higher voltage can be developed for a certain quantity of storedcharge. The higher charge-to-voltage conversion ratio can reduce theeffect of measurement errors (e.g., quantization error, comparatoroffset, etc.) introduced by subsequent quantization operation on theaccuracy of low light intensity determination. The measurement error canset a limit on a minimum voltage difference that can be detected and/ordifferentiated by the quantization operation. By increasing thecharge-to-voltage conversion ratio, the quantity of charge correspondingto the minimum voltage difference can be reduced, which in turn reducesthe lower limit of a measurable light intensity by pixel cell 602 a andextends the dynamic range.

The charge (residual charge and/or overflow charge) accumulated atcharge storage device 902 can develop an analog voltage at the OF node,which can be buffered by switchable buffer 904 at PIXEL_OUT andquantized by ADC 616. As shown in FIG. 9 , ADC 616 includes a comparator906 which can be reset by a switch M8, and output logic circuits 908.ADC 616 is also coupled with memory 912 and counter 914. Counter 914 cangenerate a set of count values based on a free-running clock signal,whereas memory 912 can be controlled, by comparator 906 via output logiccircuits 908, to store a count value (e.g., the latest count value)generated by counter 914. Memory 912 can be, for example, a latchcircuit to store the counter value based on local pixel value asdescribed below. The stored count value can be output via pixel outputbuses 816.

Comparator 906 can compare an analog voltage COMP_IN, which is derivedfrom PIXEL_OUT by the CC capacitor, against a threshold VREF, andgenerate a decision VOUT based on the comparison result. The CCcapacitor can be used in a noise/offset compensation scheme to store thereset noise and comparator offset information in a VCC voltage, whichcan be added to the PIXEL_OUT voltage to generate the COMP_IN voltage,to cancel the reset noise component in the PIXEL_OUT voltage. The offsetcomponent remains in the COMP_IN voltage and can be cancelled out by theoffset of comparator 906 when comparator 906 compares the COMP_INvoltage against threshold VREF to generate the decision VOUT. Comparator906 can generate a logical one for VOUT if the COMP_IN voltage equals orexceeds VREF. Comparator 906 can also generate a logical zero for VOUTif the COMP_IN voltage falls below VREF. VOUT can control a latch signalwhich controls memory 912 to store a count value from counter 914.

FIG. 11A illustrates an example of time-to-saturation measurement by ADC616. To perform the time-to-saturation measurement, a thresholdgenerator (which can be external to pixel cell 602 a) can generate afixed VREF. Fixed VREF can be set at a voltage corresponding a chargequantity threshold for saturation of charge storage device 902 (e.g.,threshold 1004 of FIG. 10 ). Counter 914 can start counting right afterthe exposure period starts (e.g., right after shutter switch M0 isdisabled). As the COMP_IN voltage ramps down (or up depending on theimplementation) due to accumulation of overflow charge at charge storagedevice 902, clock signal keeps toggling to update the count value atcounter 914. The COMP_IN voltage may reach the fixed VREF threshold at acertain time point, which causes VOUT to flip from low to high. Thechange of VOUT may stop the counting of counter 914, and the count valueat counter 914 may represent the time-to-saturation.

FIG. 11B illustrates an example of measurement of a quantity of chargestored at charge storage device 902. After measurement starts, thethreshold generator can generate a ramping VREF, which can either rampup (in the example of FIG. 11B) or ramp down depending onimplementation. The rate of ramping can be based on the frequency of theclock signal supplied to counter 914. In a case where overflow charge ismeasured, the voltage range of ramping VREF can be between threshold1004 (charge quantity threshold for saturation of charge storage device902) and threshold 1002 (charge quantity threshold for saturation ofphotodiode PD), which can define the medium light intensity range. In acase where residual charge is measured, the voltage range of the rampingVREF can be based on threshold 1002 and scaled by the reduced capacityof charge storage device 902 for residual charge measurement. In theexample of FIG. 11B, the quantization process can be performed withuniform quantization steps, with VREF increasing (or decreasing) by thesame amount for each clock cycle. The amount of increase (or decrease)of VREF corresponds to a quantization step. When VREF reaches within onequantization step of the COMP_IN voltage, VOUT of comparator 906 flips,which can stop the counting of counter 914, and the count value cancorrespond to a total number of quantization steps accumulated to match,within one quantization step, the COMP_IN voltage. The count value canbecome a digital representation of the quantity of charge stored atcharge storage device 902, as well as the digital representation of theincident light intensity.

As discussed above, ADC 616 can introduce quantization errors when thereis a mismatch between a quantity of charge represented by the quantitylevel output by ADC 616 (e.g., represented by the total number ofquantization steps) and the actual input quantity of charge that ismapped to the quantity level by ADC 808. The quantization error can bereduced by using a smaller quantization step size. In the example ofFIG. 11B, the quantization error can be reduced by the amount ofincrease (or decrease) in VREF per clock cycle.

Although quantization error can be reduced by using smaller quantizationstep sizes, area and performance speed may limit how far thequantization step can be reduced. With smaller quantization step size,the total number of quantization steps needed to represent a particularrange of charge quantities (and light intensity) may increase. A largernumber of data bits may be needed to represent the increased number ofquantization steps (e.g., 8 bits to represent 255 steps, 7 bits torepresent 127 steps, etc.). The larger number of data bits may requireadditional buses to be added to pixel output buses 816, which may not befeasible if pixel cell 601 is used on a head-mounted device or otherwearable devices with very limited spaces. Moreover, with a largernumber of quantization step size, ADC 808 may need to cycle through alarger number of quantization steps before finding the quantity levelthat matches (with one quantization step), which leads to increasedprocessing power consumption and time, and reduced rate of generatingimage data. The reduced rate may not be acceptable for some applicationsthat require a high frame rate (e.g., an application that tracks themovement of the eyeball).

One way to reduce quantization error is by employing a non-uniformquantization scheme, in which the quantization steps are not uniformacross the input range. FIG. 11C illustrates an example of a mappingbetween the ADC codes (the output of the quantization process) and theinput charge quantity level for a non-uniform quantization process and auniform quantization process. The dotted line illustrates the mappingfor the non-uniform quantization process, whereas the solid lineillustrates the mapping for the uniform quantization process. For theuniform quantization process, the quantization step size (denoted by Δ₁)is identical for the entire range of input charge quantity. In contrast,for the non-uniform quantization process, the quantization step sizesare different depending on the input charge quantity. For example, thequantization step size for a low input charge quantity (denoted byΔ_(S)) is smaller than the quantization step size for a large inputcharge quantity (denoted by Δ_(L)). Moreover, for the same low inputcharge quantity, the quantization step size for the non-uniformquantization process (Δ_(S)) can be made smaller than the quantizationstep size for the uniform quantization process (Δ₁).

One advantage of employing a non-uniform quantization scheme is that thequantization steps for quantizing low input charge quantities can bereduced, which in turn reduces the quantization errors for quantizingthe low input charge quantities, and the minimum input charge quantitiesthat can be differentiated by ADC 616 can be reduced. Therefore, thereduced quantization errors can push down the lower limit of themeasureable light intensity of the image sensor, and the dynamic rangecan be increased. Moreover, although the quantization errors areincreased for the high input charge quantities, the quantization errorsmay remain small compared with high input charge quantities. Therefore,the overall quantization errors introduced to the measurement of thecharge can be reduced. On the other hand, the total number ofquantization steps covering the entire range of input charge quantitiesmay remain the same (or even reduced), and the aforementioned potentialproblems associated with increasing the number of quantization steps(e.g., increase in area, reduction in processing speed, etc.) can beavoided.

FIG. 11D illustrates an example of quantizing an analog voltage by pixelADC 808 using a non-uniform quantization process. Compared with FIG. 11B(which employs a uniform quantization process), VREF increases in anon-linear fashion with each clock cycle, with a shallower slopeinitially and a steeper slope at a later time. The differences in theslopes are attributed to the uneven quantization step sizes. For lowercounter count values (which correspond to a lower input quantity range),the quantization steps are made smaller, hence VREF increases at aslower rate. For higher counter count values (which correspond to ahigher input quantity range), the quantization steps are made larger,hence VREF increases at a higher rate. The non-uniform VREF slope can begenerated based on, for example, changing the frequency of counting ofcounter 814, changing the relationship between the VREF voltage and thecount values of counter 914, etc. In some examples, the non-uniformquantization process of FIG. 11D can be employed for light intensitydetermination for low light intensity range 1006 and medium lightintensity range 1008.

Referring back to FIG. 9 , controller 920 can, based on selection 922,performs a TTS quantization operation, a quantization operation tomeasure a quantity of overflow charge (herein after, “FD ADC”operation), and a quantization operation to measure a quantity ofresidual charge (hereinafter “PD ADC” operation). Controller 920 canalso skip one or more of the quantization operations. Output logiccircuits 908 can determine which of the quantization operations to storethe count value at memory 912. Specifically, output logic circuits 908include a set of registers 932 and 934 to store the decision outputs ofthe quantization operations as FLAG_1 and FLAG_2 signals. Based on theFLAG_1 and FLAG_2 signals, controller 920 can select the count valueoutput in one of the three phases to represent the incident lightintensity. The selected count value can be stored in memory 912, andmemory 912 can be locked based on a combination of the FLAG_1 and FLAG_2signals by NOR gate 936 to prevent subsequent measurement phases fromoverwriting the selected ADC code output in memory 912. At the end ofthe three-phase measurement process, controller 920 can retrieve thecount value stored in memory 912 and provide the count value as thedigital output representing the incident light intensity.

Reference is now made to FIG. 12 , which illustrate an example sequenceof the control signals of pixel cell 602 a generated by controller 920.FIG. 12 illustrates the change of AB, RST, COMP_RST, TG, LG, and VREFwith respect to time. Referring to FIG. 12 , the period between times T0and T1 can correspond to a first reset phase, in which charge storagedevice 902 and comparator 906 can be put in a reset state by controller920 by asserting the RST and COMP_RST signals, while the shutter signalAB can be asserted to prevent charge generated by photodiode PD fromreaching charge storage device 902. Both RST and LG signals are assertedto reset C_(FD) and C_(EXT) capacitors to set PIXEL_OUT at the resetlevel. With COMP_RST signal asserted and the positive terminal ofcomparator 906 connected to V_(ref_high), COMP_IN can be set to a sum ofV_(ref_high) and comparator offset V_(comp_offset). Moreover, with RSTsignal asserted, PIXEL_OUT can be set the reset voltageV_(pixel_out_rst) and can include reset noise Vσ_(KTC). A first samplingoperation can be performed by the CC cap to store a V_(CC) voltageincluding the components of the comparator offset, the reset noise, andPIXEL_OUT voltage at the reset level, as follows:V _(cc)(T1)=(V _(ref_high) +V _(comp_offset))−(V _(pixel_out_rst) +Vσ_(KTC))  (Equation 1)

At time T1, the RST signal, the AB signal, and the COMP_RST signal arereleased, which starts an exposure period (labelled T_(exposure)) inwhich photodiode PD can accumulate and transfer charge. Exposure periodT_(exposure) can end at time T2. Between times T1 and T3, TG signal canset transfer switch M1 in a partially turned-on state to allow PD toaccumulate residual charge before photodiode PD saturates. If the lightintensity in the medium or high intensity ranges of FIG. 7 , photodiodePD can saturate and transfer overflow charge via transfer switch M1. LGsignal can remain asserted to operate in low gain mode, in which bothC_(FD) capacitor and C_(EXT) capacitor are connected in parallel to formcharge storage device 902 to store the overflow charge. The overflowcharge develops a new PIXEL_OUT voltage, V_(pixel_out_sig1). The CCcapacitor can AC couple the new PIXEL_OUT voltage V_(pixel_out_sig1)into COMP_IN voltage by adding the V_(CC) voltage, which includes thereset noise and comparator offset component. The new PIXEL_OUT voltagealso includes reset noise, which can be cancelled by the reset noisecomponent of the V_(CC) voltage. The COMP_IN voltage at time Tx betweentimes T1 and T3 can be as follows:V _(comp_in)(Tx)=V _(pixel_out_sig1) −V _(pixel_out_rst) +V _(ref_high)+V _(comp_offset)  (Equation 2)

In Equation 2, the difference betweenV_(pixel_out_sig1)−V_(pixel_out_rst) represents the quantity of overflowcharge stored in charge storage device 902. The comparator offset in theCOMP_IN voltage can also cancel out the comparator offset introduced bycomparator 906 when performing the comparison.

Between times T1 and T3, two phases of measurement of the COMP_INvoltage can be performed, including a time-to-saturation (TTS)measurement phase for high light intensity range 1010 and an FD ADCphase for measurement of overflow charge for medium light intensity1008. Between times T1 and T2 (T_(exposure)) the TTS measurement can beperformed by comparing COMP_IN voltage with a static V_(ref_low)representing a saturation level of charge storage device 902 bycomparator 906. When PIXEL_OUT voltage reaches the static VREF, theoutput of comparator 906 (VOUT) can trip, and a count value from counter914 at the time when VOUT trips can be stored into memory 912. At timeT2, controller 920 can determine the state of VOUT of comparator 906 atthe end of the TTS phase, and can assert FLAG_1 signal if VOUT isasserted. The assertion of the FLAG_1 signal can indicate that chargestorage device 902 saturates and can prevent subsequent measurementphases (FD ADC and PD ADC) from overwriting the count value stored inmemory 912. The count value from TTS can then be provided to representthe intensity of light received by the photodiode PD during theintegration period.

Between times T2 and T3 (labelled T_(FDADC)), the FD ADC operation canbe performed by comparing COMP_IN voltage with a ramping VREF voltagethat ramps from V_(ref_low) to V_(ref_high), which represents thesaturation level of photodiode PD (e.g., threshold 1002), as describedin FIG. 9B. If VOUT of comparator 906 trips during FD ADC, the countvalue of counter 914 at the time when VOUT trips can be stored in memory912, if FLAG_1 is low which indicates that charge storage device 902does not saturate. Although exposure period ends at time T2, betweentimes T2 and T3 the photodiode PD remains capable of accumulatingresidual charge (if not saturated) or transferring overflow charge tocharge storage device 902.

Between times T3 and T4 (labelled T_(PDADC-transfer)) can be the secondreset phase, in which both RST and COMP_RST signals are asserted toreset charge storage device 902 (comprising the parallel combination ofC_(FD) capacitor and C_(EXT) capacitor) and comparator 906 to preparefor the subsequent PD ADC operation. The V_(CC) voltage can be setaccording to Equation 1.

After RST and COMP_RST are released, LG is turned off to disconnectC_(EXT) from CF to increase the charge-to-voltage conversion rate forthe PD ADC operation. TG is set at a level to fully turn on the M1transfer switch to transfer the residual charge stored in the photodiodePD to Co. The residual charge develops a new PIXEL_OUT voltage,V_(pixel_out_sig2). The CC capacitor can AC couple the new PIXEL_OUTvoltage V_(pixel_out_sig2) into COMP_IN voltage by adding the V_(CC)voltage. Between times T3 and T4, the photodiode PD remains capable ofgenerating additional charge in addition to the charge generated betweentimes T1 to T3, and transferring the additional charge to charge storagedevice 902. The V_(pixel_out_sig2) also represents the additional chargetransferred between times T3 and T4. At time T4, the COMP_IN voltage canbe as follows:V _(comp_in)(T4)=V _(pixel_out_sig2) −V _(pixel_out_rst) +V _(ref_high)+V _(comp_offset)  (Equation 3)

In Equation 3, the difference betweenV_(pixel_out_sig2)−V_(pixel_out_rst) represents the quantity of chargetransferred by the photodiode to charge storage device 902 between timesT3 and T4. The comparator offset in the COMP_IN voltage can also cancelout the comparator offset introduced by comparator 906 when performingthe comparison.

At time T4, the AB signal is asserted to prevent the photodiode PD fromaccumulating and transferring additional charge. Moreover, VREF can beset a static level V_(ref_low) margin. Comparator 906 can compare theCOMP_IN voltage with V_(ref_low_margin) to determine whether thephotodiode PD saturates. V_(ref_low_margin) is slightly higher thanV_(ref_low), which represents the saturation level of photodiode PD(e.g., threshold 1002), to prevent false tripping of comparator 906 whenthe quantity of residual charge is close to but does not exceed thesaturation level. Controller 920 can determine the state of VOUT ofcomparator 906 and can assert FLAG_2 if VOUT is asserted to indicatethat photodiode PD saturates. If the FLAG_2 is asserted, memory 912 canbe locked to preserve the count value stored in memory 912 (from FD ADC)and prevents memory 912 from be overwritten by the subsequent PD ADCoperation.

Between times T4 and T5, controller 920 can perform the PD ADC operationby comparing the COMP_IN voltage with a VREF ramp that starts fromV_(ref_low) margin to V_(ref_high). In PD ADC phase, V_(ref_high) canrepresent the minimum detectable quantity of residual charge stored inphotodiode PD, whereas V_(ref_low_margin) can represent the saturationthreshold of photodiode PD with margin to account for dark current, asdescribed above. If neither FLAG_1 nor FLAG_2 is asserted prior to PDADC, the count value obtained when comparator 906 trips during PD ADCcan be stored into memory 912, and the count value from PD ADC can beprovided to represent the intensity of light.

Although FIG. 12 shows TTS, FD ADC and PD ADC operations are performed,it is understood that ADC 616 (and pixel cell 602 a) needs not performall of these operations, and can skip some of them based on selection922. As to be described below, the quantization operations may vary fordifferent photodiodes within pixel cell 602 a.

Reference is now made to FIG. 13A, which illustrates an example of pixelcell 602 a including multiple photodiodes. As shown in FIG. 13A, pixelcell 602 a includes a plurality of photodiodes including PD0, PD1, PDn,etc. PD0, PD1, and PDn can share a single charge sensing unit 614 and asingle ADC 616. Photodiodes PD0, PD1, PDn can correspond to, photodiodes612 a, 612 b, and 612 c. Each photodiode is coupled with a respectiveshutter switch and a respective transfer switch. For example, photodiodePD0 is coupled with shutter switch M0 a and transfer switch M1 a,photodiode PD1 is coupled with shutter switch M0 b and transfer switchM1 b, whereas photodiode PDn is coupled with shutter switch M0 n andtransfer switch M1 n. Each transfer switch is coupled with the OF nodeof charge storage device 902. Controller 920 can control the timing ofcontrol signals AB0, AB1, and ABn (for shutter switches M0 a, M0 b, andM0 n) and the timing of control signals TG0, TG1, and TGn (for transferswitches M1 a, M1 b, M1 n) to individually enable each photodiode togenerate/accumulate residual charge, and to transfer overflow charge tocharge sensing unit 614. In addition, based on selection 922, controller920 can also perform the quantization operations for each photodiode.

FIG. 13B illustrates example operations of pixel cell 602 a of FIG. 13A.In operation 1302, controller 920 can control shutter switches M0 a-M0 nand transfer switches M1 a-M1 n to enable each photodiode to take turnto transfer overflow charge to the single charge sensing unit togenerate the first voltage, which can then be quantized by ADC 616 in aTTS operation and/or a FD ADC operation based on selection 922. Forexample, in operation 1302, controller 920 can de-assert control signalsAB0, AB1, and ABn for shutter switches M0 a, M0 b, and M0 n to start theexposure time for photodiodes PD0, PD1, and PDn, such that eachphotodiode can generate and accumulate charge in response to a componentof the incident light. Between times T0 and T2, controller 920 canconfigure charge storage device 902 at maximum capacity, and enablephotodiode PD0 to transfer overflow charge to charge sensing unit 614 bybiasing transfer switch M1 a at the partially-on state, while disablingother transfer switches such as M1 b and M1 n to prevent otherphotodiodes from transferring overflow charge to charge sensing unit614. Based on selection 922, controller 920 can control ADC 616 toperform a TTS operation between times T0 and T1, followed by a FD ADCoperation between T1 and T2. A digital value representing the output ofthe TTS operation or the FD ADC operation for PD0 can be stored inmemory 912 based on whether charge storage device 902 (configured atmaximum capacity) saturates. Controller 920 can reset charge storagedevice 902 between times T2 and T3, and repeat the aforementionedoperations for PD1 between times T3 and T5, followed by otherphotodiodes.

After ADC 616 completes the quantization operations of the overflowcharge from each photodiode, controller 920 can allow each photodiode totake turn in transferring residual charge to charge sensing unit 614followed by a PD ADC operation. For example, between times T7 and T8controller 920 can configure charge storage device 902 at minimumcapacity, and bias transfer switch M0 a at the fully-on state totransfer the residual charge from PD0 to charge sensing unit 614,followed by a PD ADC operation to quantize the residual charge betweentimes T8 and T9. Controller 920 can reset charge storage device 902between times T9 and T10 to remove the residual charge, and then repeatthese operations for other photodiodes (e.g., PD1, PDn) after time T10.

While the arrangements of operation 1302 allow each photodiode to havethe same access to the charge sensing unit and the same set ofquantization operations can be performed on the outputs of eachphotodiode, each photodiode may have different integration periods forthe overflow charge, which can degrade the global shutter operation. Forexample, the integration period of overflow charge for photodiode PD0 isbetween times T0 and T2, whereas the integration period of overflowcharge for photodiode PD1 is between times T3 and T5. The differentintegration periods can cause motion blur as the different photodiodesmay capture light from different spots when the image sensor is movingat a high speed, which can distort the images and degrade the globalshutter operation.

In operation 1304, on the other hand, controller 920 can control shutterswitches M0 a-M0 n and transfer switches M1 a-M1 n to allow only onephotodiode to transfer overflow charge to charge sensing unit 614 toperform the TTS and FD ADC operations within the exposure period. Forexample, controller 920 can set shutter switch M0 a at a partially-onstate to allow photodiode PD0 to transfer overflow charge between timesT0 and T2, in which ADC 616 can perform TTS and FD ADC operations.Controller 920 can reset charge storage device 902 between times T2 andT3. After time T3, controller 920 can control transfer switches M1 a-M1n to allow all of the photodiodes to take turn to transfer residualcharge to the charge sensing unit and to perform the PD ADC operations.For example, between times T3 and T5, photodiode PD0 can transferresidual charge, followed by a PD ADC operation. After charge storagedevice 902 is reset between times T5 and T6, another photodiode cantransfer residual charge, followed by another PD ADC operation. Sucharrangements can be used when, for example, the intensity of aparticular component is very high compared with other components (e.g.,in a dark environment with strong infra-red illumination for 3Dsensing). The same exposure period can be provided for each photodiodeto either accumulate charge for the strong infra-red component or forthe other much weaker visible light components. The TTS and FD ADCoperations can be performed on the output of the photodiode that detectsthe strong infra-red component (e.g., photodiode PD0), while PD ADCoperations can be performed on the outputs of other photodiodes, whichcan improve the dynamic range of both the low intensity (for visiblelight) and the high intensity (for infra-red light) measurementoperations.

In some examples, controller 920 can also allow some or all thephotodiodes to transfer overflow charge or residual charge to the chargesensing unit simultaneously. Such arrangements can be part of a binningoperation, in which the transfer switches of photodiodes configureddetect the same component of incident light (e.g., photodiodes 612 a and612 b of FIG. 8D, both of which detect monochrome light) can bepartially or fully turned on to transfer overflow charge or residualcharge to charge sensing unit 614 simultaneously. Such arrangements canreduce the number of sequential quantization operations and can speed upthe processing. In some examples, controller 920 can also allowphotodiodes that detect different components of light to transferoverflow charge simultaneously to charge sensing unit 614, but set thebiases of the transfer switches to set different quantum well capacitiesfor the photodiodes. For example, the controller can lower the quantumwell capacity for a photodiode associated with a particular wavelengthrange which is expected to the be strongest among other wavelengthranges, such that that photodiode is more likely to transfer overflowcharge to the charge sensing unit than other photodiodes. Sucharrangements not only provide same exposure period for each photodiode,as in the example described above, but also enhance flexibility in thelight measurement operation. Specifically, while the TTS/FD ADCoperation output is more likely to represent the output of the expectedstrongest component of the incident light, when the operation conditionchanges and the intensities of other components also increase, theTTS/FD ADC operation output can reflect the other high intensitycomponents of the incident light as well.

In both operations 1302 and 1304, the quantization operations performedfor each photodiode can be based on selection 922. For example, inoperation 1302, it can be that only TTS operation is performed for PD0,only FD operation is performed for PD1, etc., based on selection 922.Moreover, the selection of PD0 to perform TTS and/or FD ADC operation inoperation 1304 can also be based on selection 922.

Reference is now made to FIG. 14A, which illustrates another example ofpixel cell 602 a including multiple photodiodes. As shown in FIG. 14A,pixel cell 602 a includes a plurality of photodiodes including PD0, PD1,etc., as well as a plurality of charge sensing units 614 includingcharge sensing unit 614 a, 614 b, etc. Charge sensing unit 614 aincludes a charge storage device 902 a and a switchable buffer 904 a andis configured to convert residual charge and overflow charge transferredfrom photodiode PD0 to voltages. Charge sensing unit 614 b includes acharge storage device 902 b and a switchable buffer 904 b and isconfigured to convert residual charge and overflow charge transferredform photodiode PD1 to voltages. Each photodiode is coupled with arespective shutter switch and a respective transfer switch. For example,photodiode PD0 is coupled with shutter switch M0 a and transfer switchM1 a, whereas photodiode PD1 is coupled with shutter switch M0 b andtransfer switch M1 b. Controller 920 can control the timing of controlsignals AB0 and AB1 (for shutter switches M0 a and M0 b), controlsignals TG0 and TG1 (for transfer switches M1 a and M1 b), as well ascontrol signals RST0, LG0, RST1, and LG1 to individually enable eachphotodiode to generate/accumulate residual charge, and to transferoverflow charge to a respective charge sensing unit 614. In addition,based on selection 922, controller 920 can also control the timing ofcontrol signals SEL0 and SEL1 to provide each charge sensing unit 614 aand 614 b access to ADC 616 to perform quantization operations selectedby selection 922.

FIG. 14B illustrates example operations of pixel cell 602 a of FIG. 13A.In operation 1402, controller 920 can control shutter switches M0 a-M0 nand transfer switches M1 a-M1 n to enable each photodiode to transferoverflow charge to its respective charge sensing unit simultaneously.For example, controller 920 can de-assert control signals AB0 and AB1for shutter switches M0 a and M0 b to start the exposure time forphotodiodes PD0 and PD1, such that each photodiode can generate andaccumulate charge in response to a component of the incident light.However, controller 920 only connects one of the charge sensing units toADC 616 to perform TTS operation. For example, between times T0 and T1,controller 920 can assert SEL0 to connect switchable buffer 904 ofcharge sensing unit 614 a to ADC 616 within the exposure period toperform the TTS operation. After T1, controller 920 can control eachcharge sensing unit to take turn in connecting with the ADC 616. When acharge sensing unit is connected with ADC 616, an FD ADC operation and aPD ADC operation can be performed based on selection 922. For example,as shown in operation 1402, between times T1 and T2 a FD ADC operationis performed, followed by a PD ADC operation between times T3 and T4,for photodiode PD0. Controller 920 can then assert SEL1 (whilede-asserting SEL0) to connect switchable buffer 904 of charge sensingunit 614 b to ADC 616 to perform a FD ADC operation (between times T4and T5) and a PD ADC operation (between times T6 and T7) for photodiodePD1. The arrangements of operation 1402 can be used when, for example,one of the photodiodes (e.g., PD0) is expected to receive high intensitylight component and its charge storage device is expected to besaturated as a result, while other photodiodes are expected to receivemedium/low intensity light components and their charge storage devicesare not expected to become saturated.

In operation 1404, controller 920 can also allow each photodiode to taketurn in performing the TTS operations during the exposure period, whichstarts at time T0. For example, between times T0 and T1 controller 920can assert control signal SEL0 to connect charge sensing unit 614 a toADC 616 to perform a TTS operation for PD0. Also, between times T1 andT2 controller 920 can assert control signal SEL1 to connect chargesensing unit 614 b to ADC 616 to perform a TTS operation for PD1.Controller 920 can repeat the TTS operations for other photodiodes.Starting at time T3, controller 920 can control the timing of the SEL0and SEL1 signals to allow each charge sensing unit to take turn inconnecting with ADC 616 to perform FD ADC operation and PD ADCoperations. For example, a FD ADC operation is performed for PD0 betweentimes T3 and T4, a PD ADC operation is performed for PD0 between timesT5 and T6, followed by a FD ADC operation performed for PD1 betweentimes T6 and T7, and a PD ADC operation is performed for PD1 betweentimes T7 and T8.

In both operations 1402 and 1404, the quantization operations performedfor each photodiode can be based on selection 922. For example, inoperation 1402, it can be that only TTS operation is performed for PD0,only FD operation is performed for PD1, etc., based on selection 922.Moreover, the selection of PD0 to perform TTS and/or FD ADC operation inoperation 1402 can also be based on selection 922.

In FIG. 13A-FIG. 14B, memory 912 can be configured to store thequantization results for each of the photodiodes. In some examples,memory 912 can be configured to store the quantization results for allof the photodiodes simultaneously. In some examples, the quantizationresults of each photodiode can be stored in memory 912 sequentially.Specifically, the quantization results of one photodiode of the pixelcell can be stored in the memory, read out for generation of the pixelvalue, and then overwritten by the quantization results of anotherphotodiode of the pixel cell.

Although FIG. 14A illustrates an ADC 616 being shared among a pluralityof charge sensing units, it is understood that a pixel cell can includemultiple ADCs shared by subsets of charge sensing units. For example, ina case where a pixel cell includes four photodiodes and four chargesensing units, each pair of the charge sensing units can share one ADC,and the pixel cell can include two ADCs. Such arrangements can reducethe number of sequential quantization operations to be performed by eachADC, which can reduce the time it takes to generate the digital outputsand can increase the frame rate.

FIG. 15 illustrates a flowchart of a method 1500 for measuringintensities of different components of light. Method 1500 can beperformed by, for example, various components of image sensor 600including pixel cell 602 a, memory 912, counter 914, and controller 920as shown in FIG. 13A and FIG. 14A.

Method 1500 starts with step 1502, in which controller 920 can enableeach photodiode of a plurality of photodiodes (e.g., PD0, PD1, PDn,etc.) of pixel cell 602 a to generate charge in response to a differentcomponent of the incident light. As described above, pixel cell 602 amay include a plurality of photodiodes. In some examples, as shown inFIG. 8A, the photodiodes can be stacked to have different distances froma light receiving surface which can set the wavelength of the lightcomponent absorbed by each photodiode. In some examples, as shown inFIGS. 8B-8C, the photodiodes can be arranged laterally to formsub-pixels of the pixel cell and can be overlaid with one or moremicrolens and one or more optical filter arrays to select the wavelengthof the light component received by each photodiode. The controller canenable each photodiode to generate charge in response to the respectivelight component by disabling the shutter switches (e.g., M0 a, M0 b,etc.) coupled with each photodiode.

In step 1504, the controller can transfer the charge generated by theplurality of photodiodes to one or more charge sensing units 614 toconvert to voltages. The charge may include residual charge accumulatedat each photodiode, as well as overflow charge transferred by the eachphotodiode after the photodiode saturates. In some examples, as shown inFIG. 14A, the photodiodes share a single charge sensing unit 614. Eachphotodiode is coupled with the single charge sensing unit via a transferswitch (e.g., M1 a, M1 b, M1 n, etc.). The controller can control thetransfer switches to control a timing of when each photodiode transfersthe overflow charge to the single charge sensing unit, as described inFIG. 13B. In some examples, as shown in FIG. 14B, each photodiode mayhave a corresponding charge sensing unit 614, and the controller canallow each photodiode to transfer overflow charge to its respectivecharge sensing unit 614 simultaneously.

In step 1506, the controller can receive, for each photodiode of theplurality of photodiodes, a selection of one or more quantizationprocesses of a plurality of quantization processes corresponding to aplurality of intensity ranges. The plurality of quantization processesmay include, for example, a time-to-saturation (TTS) measurementoperation to measure a time for the charge sensing unit to becomesaturated by the overflow charge, a FD ADC measurement operation tomeasure a quantity of overflow charge, and a PD ADC measurementoperation to measure a quantity of residual charge. The TTS operationcan be for a high light intensity range, the FD ADC measurementoperation can be for a medium light intensity range, whereas the PD ADCmeasurement operation can be for a low light intensity range.

The selection of the quantization operations may be specific to, forexample, the sharing of the charge sensing units among the photodiodes.For example, referring to FIG. 13B, the selection may include selectingeach photodiode to perform, at different times, TTS and FD ADCoperations based on the overflow charge, followed by residual chargetransfer and PD ADC operation for each photodiode. The selection mayalso include selecting one of the photodiodes to perform TTS and FD ADCoperation based on overflow charge from that photodiode, followed byresidual charge transfer and PD ADC operation for each photodiodes. Asanother example, referring to FIG. 14B, the selection may includeselecting each photodiode to take turn in performing TTS, FD ADC, and PDADC operation, selecting each photodiode to take turn in performing TTSoperations, followed by FD ADC and PD ADC operations.

In step 1508, the controller can control one or more ADCs 616 to performthe selected quantization operations to quantize the voltages from theone or more charge sensing units to digital values representingcomponents of a pixel of different wavelength ranges. For example, adigital value can be generated for one of the photodiodes based on theTTS, FD ADC, and PD ADC operations, whereas a digital value can begenerated for other photodiodes based on the FD ADC and PD ADCoperations. The quantization can be performed by a comparator (e.g.,comparator 906) comparing the voltages with a static threshold (for TTS)or a ramping threshold (for FD ADC and PD ADC) to control when memory912 stores a count value from counter 914. The count value can be thedigital values.

In step 1510, the controller can store at least some of the digitalvalues in memory 912. In some examples, memory 912 may have sufficientcapacity to store a digital value for each photodiode representing acomponent of the pixel. In some examples, memory 912 may store thedigital value of one photodiode, output the digital value to an imagingmodule (e.g., an imaging module 628), and then store the digital valueof another photodiode. The imaging module can acquire the digital valuesof the pixel and construct the pixel based on the digital values, instep 1512.

Some portions of this description describe the examples of thedisclosure in terms of algorithms and symbolic representations ofoperations on information. These algorithmic descriptions andrepresentations are commonly used by those skilled in the dataprocessing arts to convey the substance of their work effectively toothers skilled in the art. These operations, while describedfunctionally, computationally, or logically, are understood to beimplemented by computer programs or equivalent electrical circuits,microcode, or the like. Furthermore, it has also proven convenient attimes, to refer to these arrangements of operations as modules, withoutloss of generality. The described operations and their associatedmodules may be embodied in software, firmware, and/or hardware.

Steps, operations, or processes described may be performed orimplemented with one or more hardware or software modules, alone or incombination with other devices. In some examples, a software module isimplemented with a computer program product comprising acomputer-readable medium containing computer program code, which can beexecuted by a computer processor for performing any or all of the steps,operations, or processes described.

Examples of the disclosure may also relate to an apparatus forperforming the operations described. The apparatus may be speciallyconstructed for the required purposes, and/or it may comprise ageneral-purpose computing device selectively activated or reconfiguredby a computer program stored in the computer. Such a computer programmay be stored in a non-transitory, tangible computer readable storagemedium, or any type of media suitable for storing electronicinstructions, which may be coupled to a computer system bus.Furthermore, any computing systems referred to in the specification mayinclude a single processor or may be architectures employing multipleprocessor designs for increased computing capability.

Examples of the disclosure may also relate to a product that is producedby a computing process described herein. Such a product may compriseinformation resulting from a computing process, where the information isstored on a non-transitory, tangible computer readable storage mediumand may include any example of a computer program product or other datacombination described herein.

The language used in the specification has been principally selected forreadability and instructional purposes, and it may not have beenselected to delineate or circumscribe the inventive subject matter. Itis therefore intended that the scope of the disclosure be limited not bythis detailed description, but rather by any claims that issue on anapplication based hereon. Accordingly, the disclosure of the examples isintended to be illustrative, but not limiting, of the scope of thedisclosure, which is set forth in the following claims.

What is claimed is:
 1. An apparatus comprising: multiple distinct setsof photodiodes, each set of photodiodes in the multiple distinct sets ofphotodiodes being configured to convert a component of incident light ofa unique wavelength range to charge, wherein each set of photodiodescomprises one or more photodiodes; one or more charge sensing units; anda controller configured to: transfer charge generated by the one or morephotodiodes in response to a different component of incident light tothe one or more charge sensing units in order to convert the charge tovoltages; perform one or more quantization processes of a plurality ofquantization processes corresponding to a plurality of intensity ranges,wherein the one or more quantization processes quantizes the voltagesfrom the one or more charge sensing units to digital values representingcomponents of a pixel of different wavelength ranges; and generate apixel value based on the at least some of the digital values.
 2. Theapparatus of claim 1, wherein the one or more photodiodes are configuredto, within an integration period, accumulate at least a part of thecharge as residual charge until the one or more photodiodes saturates,and to transfer the remaining charge as overflow charge to the one ormore charge sensing units after the one or more photodiode saturates;wherein the one or more charge sensing units comprises a charge storagedevice having a configurable capacitance; wherein the plurality ofquantization processes comprise: a first quantization operation togenerate a first digital value representing a quantity of the overflowcharge received by the charge storage device configured at a maximumcapacitance, the first quantization operating being associated with afirst intensity range; and a second quantization operation to, after theresidual charge is transferred to the charge storage device configuredat a minimum capacitance, generate a second digital value representing aquantity of the residual charge stored at the charge storage device, thesecond quantization operation being associated with a second intensityrange lower than the first intensity range.
 3. The apparatus of claim 2,wherein the plurality of quantization processes comprises a thirdquantization operation to generate a third digital value representing atime-of-saturation of the charge storage device caused by the overflowcharge.
 4. The apparatus of claim 3, further comprising a lightreceiving surface through which the plurality of the photodiodes receivethe incident light; wherein the multiple distinct sets of photodiodesform a stack structure with respect to the light receiving surface suchthat the one or more photodiodes are separated from the light receivingsurface by a different distance; and wherein the component converted bythe one or more photodiodes is based on the respective distance betweenthe one or more photodiodes and the light receiving surface.
 5. Theapparatus of claim 3, further comprising: a light receiving surface; anda filter array on a first side of the light receiving surface, thefilter array having filter elements positioned at a plurality oflocations on the first side of the light receiving surface to set acomponent of the incident light that enters the light receiving surfaceat the respective location, wherein multiple distinct sets ofphotodiodes correspond to a plurality of sub-pixels and are positionedat the plurality of locations on a second side of the light receivingsurface to receive the respective components of the incident light, andwherein multiple distinct sets of photodiodes convert incident light ofa respective wavelength range based on the filter array.
 6. Theapparatus of claim 5, further comprising a single microlens over aplurality of filter arrays including the filter array and configured toproject the incident light received from one spot of a scene towards theplurality of locations on the first side of the light receiving surface.7. The apparatus of claim 5, further comprising a plurality ofmicrolenses including a first microlens, the first microlens coveringthe filter array and configured to project the incident light receivedfrom one spot of a scene towards the plurality of locations on the firstside of the light receiving surface.
 8. The apparatus of claim 3,wherein the one or more charge sensing units comprises a single chargesensing unit and the apparatus further comprises an analog-to-digitalconverter (ADC); wherein the ADC is coupled with an output of the singlecharge sensing unit; wherein the apparatus further comprises a pluralityof switches, each switch coupled between a photodiode and an input ofthe single charge sensing unit; wherein the controller is configured to:control the plurality of switches to transfer the charge generated bythe one or more photodiodes to the single charge sensing unit to convertto voltages; and control the ADC to quantize the voltages generated bythe single charge sensing unit.
 9. The apparatus of claim 8, wherein thecontroller is configured to: control a first switch of the plurality ofswitches to transfer a first overflow charge from a first photodiode ofthe set of photodiodes to the single charge sensing unit to convert to afirst voltage; control the ADC to perform at least one of the first orthird quantization processes of the first voltage to generate a firstdigital value; control the first switch to transfer a first residualcharge from the first photodiode to the single charge sensing unit toconvert to a second voltage; control the ADC to perform the secondquantization operation of the second voltage to generate a seconddigital value; control a second switch of the plurality of switches totransfer a second residual charge from a second photodiode of the set ofphotodiodes to the single charge sensing unit to convert to a thirdvoltage; control the ADC to perform the second quantization operation ofthe third voltage to generate a third digital value; and generate thepixel value based on one of first digital value and the second digitalvalue.
 10. The apparatus of claim 8, wherein each of the photodiodes hasa different full well capacity for storing the residual charge; andwherein the controller is configured to: control the plurality ofswitches to transfer overflow charge from the one or more photodiodes ofthe set of photodiodes to the single charge sensing unit simultaneouslyto generate a first voltage; control the ADC to quantize the firstvoltage using at least one of the first or third quantization processesto generate a first digital value; control the plurality of switches totransfer residual charge from the one or more photodiodes to the singlecharge sensing unit at different times to generate second voltages eachcorresponding the respective residual charge from the one or morephotodiodes; control the ADC to quantize the second voltages using thesecond quantization operation to generate second digital values; andgenerate the pixel value based on the first digital value and the seconddigital values.
 11. The apparatus of claim 8, wherein the controller isconfigured to: within a first time period: control the plurality ofswitches to transfer overflow charge from the one or more photodiodes ofthe set of photodiodes to the single charge sensing unit at differenttimes to generate first voltages each corresponding the respectiveoverflow charge from the one or more photodiodes; and control the ADC toquantize the first voltages using at least one of the first or thirdquantization processes to generate first digital values; within a secondtime period: control the plurality of switches to transfer residualcharge from the one or more photodiodes to the single charge sensingunit at different times to generate second voltages each correspondingto the respective residual charge from the one or more photodiodes;control the ADC to quantize the second voltages using the secondquantization operation to generate second digital values; and generatethe pixel value based on at least some of the first digital values andthe second digital values.
 12. The apparatus of claim 3, wherein the oneor more charge sensing units comprises a plurality of charge sensingunits corresponding to the set of photodiodes; and wherein the apparatusfurther comprises a plurality of switches each coupled between eachcharge sensing unit of the plurality of charge sensing units and acorresponding photodiode of the set of photodiodes.
 13. The apparatus ofclaim 12, wherein the controller is configured to: enable a firstphotodiode of the plurality of the photodiodes to transfer a firstcharge to a first charge sensing unit of the plurality of charge sensingunits to generate a first voltage; enable a second photodiode of theplurality of the photodiodes to transfer a second charge to a secondcharge sensing unit of the plurality of charge sensing units to generatea second voltage; perform the first quantization operation of the firstvoltage to generate a first digital value, followed by the second orthird quantization processes of the second voltage to generate a seconddigital value; and generate the pixel value based on the first digitalvalue and the second digital value.
 14. The apparatus of claim 13,wherein the controller is configured to: within a first time period:enable a first photodiode of the plurality of the photodiodes togenerate a first charge in response to the incident light; enable thefirst photodiode to transfer a first overflow charge of the first chargeto a first charge sensing unit of the plurality of charge sensing unitsto generate a first voltage; and perform the third quantizationoperation of the first voltage to generate a first digital valuerepresenting a first time-to-saturation; within a second time period:enable a second photodiode of the plurality of the photodiodes togenerate a second charge in response to the incident light; enable thesecond photodiode to transfer a second overflow charge of the secondcharge to a second charge sensing unit of the plurality of chargesensing units to generate a second voltage; and perform the thirdquantization operation of the second voltage to generate a seconddigital value representing a second time-to-saturation; and generate thepixel value based on the first digital value and the second digitalvalue.
 15. The apparatus of claim 12, wherein the plurality of chargesensing units comprises a first charge sensing unit, a second chargesensing unit, a third charge sensing unit, and a fourth charge sensingunit; wherein apparatus comprises a first ADC and a second ADC; andwherein the controller is configured to: control the first ADC toquantize a first voltage from the first charge sensing unit and a secondvoltage from the second charge sensing unit; and control the second ADCto quantize a third voltage from the third charge sensing unit and athird voltage from the second charge sensing unit.
 16. The apparatus ofclaim 1, wherein the controller is configured to store each of thedigital values in a memory.
 17. The apparatus of claim 1, wherein theapparatus comprises a memory and wherein the controller is configuredto: generate a first digital value based on quantizing a first voltagecorresponding to charge generated by a first photodiode of the set ofphotodiodes; store the first digital value in the memory; read the firstdigital value to compute the pixel value; generate a second digitalvalue based on quantizing a second voltage corresponding to chargegenerated by a second photodiode of the set of photodiodes; overwritethe first digital value with a second digital value in the memory; andread the second digital value to compute the pixel value.
 18. A methodcomprising: enabling one or more photodiodes within a set of photodiodesto generate charge in response to a different component of incidentlight, wherein the set of photodiodes is part of multiple distinct setsof photodiodes, wherein each set of photodiodes is configured to converta component of incident light of a unique wavelength range to charge;transferring the charge from the one or more photodiodes to one or morecharge sensing units; controlling performance of one or morequantization processes to quantize voltages from the one or more chargesensing units to digital values representing components of a pixel ofdifferent wavelength ranges; and generating a pixel value based on theat least some of the digital values.
 19. The method of claim 18, whereinplurality of quantization processes comprises a first quantizationprocess to measure a quantity of residual charge accumulated at a firstphotodiode within the set of photodiodes before the first photodiodesaturates, a second quantization process to measure a quantity ofoverflow charge transferred by the first photodiode after the firstphotodiode saturates, and a third quantization process to measure atime-to-saturation of the one or more charge sensing units caused by theoverflow charge from the first photodiode.
 20. The method of claim 18,wherein the one or more charge sensing units comprises a single chargesensing unit shared by the set of photodiodes in the multiple distinctsets of photodiodes.